docs: document monthly view, updated layout, and deferred work

Refs #3

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

README.md

@@ -2,19 +2,22 @@
 
 Household budget tracker. FastAPI + HTMX frontend, SQLite backend.
 
-## Sections
+## Pages
 
-The budget page shows one card per section. Every section accepts a name and
+* `/` budget configuration. One section per category (Incomes, Fixed Amount
-amount per entry and displays a running total, except *Primary Debt Target*,
+  Bills, Debt Minimums, Primary Debt Target, Food and Essentials,
-which is a pointer to one of the *Debt Minimums* rows.
+  Subscriptions, Other). Every section accepts name + amount entries and
+  shows a running total. The Primary Debt Target is a pointer to a Debt
+  Minimums row.
+* `/month/YYYY-MM` monthly view. Snapshots the budget at creation time and
+  tracks an `applied` amount per entry alongside the planned amount. Each
+  row is annotated when its name or planned value has been edited away
+  from the snapshot, or when the row was added after creation. Per-month
+  debt target is independent of the budget's target after snapshot.
 
-* Incomes
+Navigate between months with the prev / next buttons or the dropdown
-* Fixed Amount Bills
+picker. A "This month" link on `/` jumps to the current `YYYY-MM`; if it
-* Debt Minimums
+has not been created yet, you land on the create flow.
-* Primary Debt Target (pointer)
-* Food and Essentials
-* Subscriptions
-* Other
 
 ## Requirements
 
@@ -51,19 +54,22 @@ Tests run against an in-memory SQLite database; no migration step needed.
 
 ```
 src/quartermaster/
-  main.py         FastAPI app factory
+  main.py            FastAPI app factory
-  routes.py       HTTP handlers, HTMX partials
+  routes.py          Budget configuration HTTP handlers
-  service.py      Queries, totals, target logic
+  routes_month.py    Monthly view HTTP handlers
-  models.py       SQLAlchemy models and Section enum
+  service.py         Budget queries, totals, target logic
-  db.py           Engine, session, PRAGMA foreign_keys=ON
+  month_service.py   Snapshot, deviation, per-month CRUD
-  config.py       DB URL resolution
+  models.py          SQLAlchemy models and Section enum
-  templates/      Jinja2 templates (base, index, partials)
+  db.py              Engine, session, PRAGMA foreign_keys=ON
-  static/         CSS
+  config.py          DB URL resolution
-alembic/          Migrations
+  templates/         Jinja2 templates (base, index, month, partials)
-tests/            pytest suite
+  static/            CSS
+alembic/             Migrations
+tests/               pytest suite
 ```
 
-## Scope
+## Deferred
 
-Single-month budget. Multi-month support is planned for a later milestone
+A transaction log that rolls up into `applied` on a per-entry per-month
-and is intentionally not modelled yet.
+basis is deferred. Once implemented it may replace the hand-edited applied
+field.

alembic/versions/03ebe3c07262_add_month_snapshot_tables.py

@@ -0,0 +1,73 @@
+"""add month snapshot tables
+
+Revision ID: 03ebe3c07262
+Revises: f1ccdc4bc1bf
+Create Date: 2026-04-17 11:33:45.853510
+
+"""
+from typing import Sequence, Union
+
+from alembic import op
+import sqlalchemy as sa
+
+
+# revision identifiers, used by Alembic.
+revision: str = '03ebe3c07262'
+down_revision: Union[str, Sequence[str], None] = 'f1ccdc4bc1bf'
+branch_labels: Union[str, Sequence[str], None] = None
+depends_on: Union[str, Sequence[str], None] = None
+
+
+def upgrade() -> None:
+    """Upgrade schema."""
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.create_table('month',
+    sa.Column('id', sa.Integer(), nullable=False),
+    sa.Column('year_month', sa.String(length=7), nullable=False),
+    sa.Column('created_at', sa.DateTime(timezone=True), server_default=sa.text('(CURRENT_TIMESTAMP)'), nullable=False),
+    sa.PrimaryKeyConstraint('id'),
+    sa.UniqueConstraint('year_month')
+    )
+    op.create_table('month_entry',
+    sa.Column('id', sa.Integer(), nullable=False),
+    sa.Column('month_id', sa.Integer(), nullable=False),
+    sa.Column('section', sa.Enum('income', 'fixed_bill', 'debt_minimum', 'food', 'subscription', 'other', name='section', native_enum=False, length=32), nullable=False),
+    sa.Column('name', sa.String(length=128), nullable=False),
+    sa.Column('planned', sa.Numeric(precision=10, scale=2), nullable=False),
+    sa.Column('applied', sa.Numeric(precision=10, scale=2), server_default='0.00', nullable=False),
+    sa.Column('origin_name', sa.String(length=128), nullable=True),
+    sa.Column('origin_planned', sa.Numeric(precision=10, scale=2), nullable=True),
+    sa.Column('source_entry_id', sa.Integer(), nullable=True),
+    sa.Column('created_at', sa.DateTime(timezone=True), server_default=sa.text('(CURRENT_TIMESTAMP)'), nullable=False),
+    sa.Column('updated_at', sa.DateTime(timezone=True), server_default=sa.text('(CURRENT_TIMESTAMP)'), nullable=False),
+    sa.ForeignKeyConstraint(['month_id'], ['month.id'], ondelete='CASCADE'),
+    sa.ForeignKeyConstraint(['source_entry_id'], ['entry.id'], ondelete='SET NULL'),
+    sa.PrimaryKeyConstraint('id'),
+    sa.UniqueConstraint('month_id', 'id')
+    )
+    with op.batch_alter_table('month_entry', schema=None) as batch_op:
+        batch_op.create_index(batch_op.f('ix_month_entry_month_id'), ['month_id'], unique=False)
+        batch_op.create_index(batch_op.f('ix_month_entry_section'), ['section'], unique=False)
+
+    op.create_table('month_debt_target',
+    sa.Column('month_id', sa.Integer(), autoincrement=False, nullable=False),
+    sa.Column('month_entry_id', sa.Integer(), nullable=True),
+    sa.Column('updated_at', sa.DateTime(timezone=True), server_default=sa.text('(CURRENT_TIMESTAMP)'), nullable=False),
+    sa.ForeignKeyConstraint(['month_entry_id'], ['month_entry.id'], ondelete='SET NULL'),
+    sa.ForeignKeyConstraint(['month_id'], ['month.id'], ondelete='CASCADE'),
+    sa.PrimaryKeyConstraint('month_id')
+    )
+    # ### end Alembic commands ###
+
+
+def downgrade() -> None:
+    """Downgrade schema."""
+    # ### commands auto generated by Alembic - please adjust! ###
+    op.drop_table('month_debt_target')
+    with op.batch_alter_table('month_entry', schema=None) as batch_op:
+        batch_op.drop_index(batch_op.f('ix_month_entry_section'))
+        batch_op.drop_index(batch_op.f('ix_month_entry_month_id'))
+
+    op.drop_table('month_entry')
+    op.drop_table('month')
+    # ### end Alembic commands ###

src/quartermaster/main.py

@@ -6,6 +6,7 @@ from fastapi import FastAPI
 from fastapi.staticfiles import StaticFiles
 
 from quartermaster.routes import router
+from quartermaster.routes_month import router as month_router
 
 STATIC_DIR = Path(__file__).parent / "static"
 
@@ -14,6 +15,7 @@ def create_app() -> FastAPI:
     app = FastAPI(title="Quartermaster", version="0.1.0")
     app.mount("/static", StaticFiles(directory=str(STATIC_DIR)), name="static")
     app.include_router(router)
+    app.include_router(month_router)
     return app
 
 

src/quartermaster/models.py

@@ -11,6 +11,7 @@ from sqlalchemy import (
     ForeignKey,
     Numeric,
     String,
+    UniqueConstraint,
     func,
 )
 from sqlalchemy.orm import DeclarativeBase, Mapped, mapped_column, relationship
@@ -75,3 +76,82 @@ class DebtTarget(Base):
     )
 
     entry: Mapped[Entry | None] = relationship(Entry, lazy="joined")
+
+
+class Month(Base):
+    __tablename__ = "month"
+
+    id: Mapped[int] = mapped_column(primary_key=True)
+    year_month: Mapped[str] = mapped_column(String(7), nullable=False, unique=True)
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), server_default=func.now(), nullable=False
+    )
+
+    entries: Mapped[list["MonthEntry"]] = relationship(
+        back_populates="month", cascade="all, delete-orphan"
+    )
+    target: Mapped["MonthDebtTarget | None"] = relationship(
+        back_populates="month",
+        cascade="all, delete-orphan",
+        uselist=False,
+        lazy="joined",
+    )
+
+
+class MonthEntry(Base):
+    __tablename__ = "month_entry"
+    __table_args__ = (UniqueConstraint("month_id", "id"),)
+
+    id: Mapped[int] = mapped_column(primary_key=True)
+    month_id: Mapped[int] = mapped_column(
+        ForeignKey("month.id", ondelete="CASCADE"), nullable=False, index=True
+    )
+    section: Mapped[Section] = mapped_column(
+        Enum(Section, native_enum=False, length=32), nullable=False, index=True
+    )
+    name: Mapped[str] = mapped_column(String(128), nullable=False)
+    planned: Mapped[Decimal] = mapped_column(Numeric(10, 2), nullable=False)
+    applied: Mapped[Decimal] = mapped_column(
+        Numeric(10, 2), nullable=False, default=Decimal("0.00"),
+        server_default="0.00",
+    )
+    origin_name: Mapped[str | None] = mapped_column(String(128), nullable=True)
+    origin_planned: Mapped[Decimal | None] = mapped_column(
+        Numeric(10, 2), nullable=True
+    )
+    source_entry_id: Mapped[int | None] = mapped_column(
+        ForeignKey("entry.id", ondelete="SET NULL"), nullable=True
+    )
+    created_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True), server_default=func.now(), nullable=False
+    )
+    updated_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True),
+        server_default=func.now(),
+        onupdate=func.now(),
+        nullable=False,
+    )
+
+    month: Mapped[Month] = relationship(back_populates="entries")
+
+
+class MonthDebtTarget(Base):
+    __tablename__ = "month_debt_target"
+
+    month_id: Mapped[int] = mapped_column(
+        ForeignKey("month.id", ondelete="CASCADE"),
+        primary_key=True,
+        autoincrement=False,
+    )
+    month_entry_id: Mapped[int | None] = mapped_column(
+        ForeignKey("month_entry.id", ondelete="SET NULL"), nullable=True
+    )
+    updated_at: Mapped[datetime] = mapped_column(
+        DateTime(timezone=True),
+        server_default=func.now(),
+        onupdate=func.now(),
+        nullable=False,
+    )
+
+    month: Mapped[Month] = relationship(back_populates="target")
+    entry: Mapped[MonthEntry | None] = relationship(MonthEntry, lazy="joined")

src/quartermaster/month_service.py

@@ -0,0 +1,228 @@
+from __future__ import annotations
+
+import re
+from dataclasses import dataclass
+from datetime import date
+from decimal import Decimal
+from enum import Enum
+
+from sqlalchemy import select
+from sqlalchemy.orm import Session
+
+from quartermaster.models import (
+    DebtTarget,
+    Entry,
+    Month,
+    MonthDebtTarget,
+    MonthEntry,
+    Section,
+)
+
+YEAR_MONTH_RE = re.compile(r"^\d{4}-(0[1-9]|1[0-2])$")
+
+
+class DeviationState(str, Enum):
+    unchanged = "unchanged"
+    edited = "edited"
+    new_in_month = "new_in_month"
+
+
+@dataclass(frozen=True)
+class MonthRow:
+    entry: MonthEntry
+    state: DeviationState
+
+
+@dataclass(frozen=True)
+class MonthSectionView:
+    section: Section
+    label: str
+    rows: list[MonthRow]
+    total_planned: Decimal
+    total_applied: Decimal
+
+
+def valid_year_month(year_month: str) -> bool:
+    return bool(YEAR_MONTH_RE.match(year_month))
+
+
+def current_year_month() -> str:
+    today = date.today()
+    return f"{today.year:04d}-{today.month:02d}"
+
+
+def shift_year_month(year_month: str, delta: int) -> str:
+    year, month = (int(part) for part in year_month.split("-"))
+    index = (year * 12 + (month - 1)) + delta
+    new_year, new_month0 = divmod(index, 12)
+    return f"{new_year:04d}-{new_month0 + 1:02d}"
+
+
+def get_month(db: Session, year_month: str) -> Month | None:
+    stmt = select(Month).where(Month.year_month == year_month)
+    return db.scalar(stmt)
+
+
+def list_months(db: Session) -> list[Month]:
+    stmt = select(Month).order_by(Month.year_month)
+    return list(db.scalars(stmt))
+
+
+def create_month(db: Session, year_month: str) -> Month:
+    if not valid_year_month(year_month):
+        raise ValueError("year_month must be formatted as YYYY-MM")
+    existing = get_month(db, year_month)
+    if existing is not None:
+        return existing
+
+    month = Month(year_month=year_month)
+    db.add(month)
+    db.flush()
+
+    budget_entries = list(db.scalars(select(Entry).order_by(Entry.id)))
+    source_to_month_entry: dict[int, MonthEntry] = {}
+    for e in budget_entries:
+        month_entry = MonthEntry(
+            month_id=month.id,
+            section=e.section,
+            name=e.name,
+            planned=e.amount,
+            applied=Decimal("0.00"),
+            origin_name=e.name,
+            origin_planned=e.amount,
+            source_entry_id=e.id,
+        )
+        db.add(month_entry)
+        source_to_month_entry[e.id] = month_entry
+
+    db.flush()
+
+    budget_target = db.get(DebtTarget, 1)
+    target_entry_id: int | None = None
+    if budget_target is not None and budget_target.debt_minimum_id is not None:
+        mapped = source_to_month_entry.get(budget_target.debt_minimum_id)
+        if mapped is not None:
+            target_entry_id = mapped.id
+
+    db.add(
+        MonthDebtTarget(month_id=month.id, month_entry_id=target_entry_id)
+    )
+    db.commit()
+    db.refresh(month)
+    return month
+
+
+def deviation_state(entry: MonthEntry) -> DeviationState:
+    if entry.origin_name is None or entry.origin_planned is None:
+        return DeviationState.new_in_month
+    if entry.name != entry.origin_name or entry.planned != entry.origin_planned:
+        return DeviationState.edited
+    return DeviationState.unchanged
+
+
+def _rows(entries: list[MonthEntry]) -> list[MonthRow]:
+    return [MonthRow(entry=e, state=deviation_state(e)) for e in entries]
+
+
+def section_view(month: Month, section: Section, label: str) -> MonthSectionView:
+    entries = [e for e in month.entries if e.section == section]
+    entries.sort(key=lambda e: e.id)
+    rows = _rows(entries)
+    total_planned = sum((e.planned for e in entries), Decimal("0"))
+    total_applied = sum((e.applied for e in entries), Decimal("0"))
+    return MonthSectionView(
+        section=section,
+        label=label,
+        rows=rows,
+        total_planned=total_planned,
+        total_applied=total_applied,
+    )
+
+
+def add_month_entry(
+    db: Session,
+    month: Month,
+    section: Section,
+    name: str,
+    planned: Decimal,
+) -> MonthEntry:
+    entry = MonthEntry(
+        month_id=month.id,
+        section=section,
+        name=name.strip(),
+        planned=planned,
+        applied=Decimal("0.00"),
+        origin_name=None,
+        origin_planned=None,
+        source_entry_id=None,
+    )
+    db.add(entry)
+    db.commit()
+    db.refresh(entry)
+    return entry
+
+
+def get_month_entry(db: Session, month: Month, entry_id: int) -> MonthEntry | None:
+    entry = db.get(MonthEntry, entry_id)
+    if entry is None or entry.month_id != month.id:
+        return None
+    return entry
+
+
+def delete_month_entry(db: Session, month: Month, entry_id: int) -> Section | None:
+    entry = get_month_entry(db, month, entry_id)
+    if entry is None:
+        return None
+    section = entry.section
+    db.delete(entry)
+    db.commit()
+    return section
+
+
+def update_month_entry(
+    db: Session,
+    month: Month,
+    entry_id: int,
+    *,
+    name: str | None = None,
+    planned: Decimal | None = None,
+    applied: Decimal | None = None,
+) -> MonthEntry | None:
+    entry = get_month_entry(db, month, entry_id)
+    if entry is None:
+        return None
+    if name is not None:
+        entry.name = name.strip()
+    if planned is not None:
+        entry.planned = planned
+    if applied is not None:
+        entry.applied = applied
+    db.commit()
+    db.refresh(entry)
+    return entry
+
+
+def get_month_target(db: Session, month: Month) -> MonthDebtTarget:
+    if month.target is not None:
+        return month.target
+    target = MonthDebtTarget(month_id=month.id, month_entry_id=None)
+    db.add(target)
+    db.commit()
+    db.refresh(target)
+    return target
+
+
+def set_month_target(
+    db: Session, month: Month, month_entry_id: int | None
+) -> MonthDebtTarget:
+    target = get_month_target(db, month)
+    if month_entry_id is not None:
+        candidate = get_month_entry(db, month, month_entry_id)
+        if candidate is None or candidate.section != Section.debt_minimum:
+            raise ValueError(
+                "month_entry_id must reference a debt minimum entry in this month"
+            )
+    target.month_entry_id = month_entry_id
+    db.commit()
+    db.refresh(target)
+    return target

src/quartermaster/routes.py

@@ -8,7 +8,7 @@ from fastapi.responses import HTMLResponse
 from fastapi.templating import Jinja2Templates
 from sqlalchemy.orm import Session
 
-from quartermaster import service
+from quartermaster import month_service, service
 from quartermaster.db import get_session
 from quartermaster.models import SECTION_LABELS, Section
 
@@ -62,6 +62,7 @@ def index(request: Request, db: Session = Depends(get_session)) -> HTMLResponse:
     sections = [_section_view(db, s) for s in Section]
     target = service.get_debt_target(db)
     debt_minimums = service.list_entries(db, Section.debt_minimum)
+    current_ym = month_service.current_year_month()
     return templates.TemplateResponse(
         request,
         "index.html",
@@ -69,6 +70,8 @@ def index(request: Request, db: Session = Depends(get_session)) -> HTMLResponse:
             "sections": sections,
             "target": target,
             "debt_minimums": debt_minimums,
+            "current_year_month": current_ym,
+            "all_months": month_service.list_months(db),
         },
     )
 

src/quartermaster/routes_month.py

@@ -0,0 +1,224 @@
+from __future__ import annotations
+
+from decimal import Decimal, InvalidOperation
+
+from fastapi import APIRouter, Depends, Form, HTTPException, Request
+from fastapi.responses import HTMLResponse, Response
+from sqlalchemy.orm import Session
+
+from quartermaster import month_service, service
+from quartermaster.db import get_session
+from quartermaster.models import SECTION_LABELS, Month, Section
+from quartermaster.routes import templates
+
+router = APIRouter(prefix="/month/{year_month}", tags=["month"])
+
+
+def _parse_amount(raw: str, *, allow_zero: bool = True) -> Decimal:
+    try:
+        amount = Decimal(raw.strip())
+    except (InvalidOperation, AttributeError) as exc:
+        raise HTTPException(status_code=400, detail="amount must be numeric") from exc
+    if amount < 0:
+        raise HTTPException(status_code=400, detail="amount must be non-negative")
+    if amount == 0 and not allow_zero:
+        raise HTTPException(status_code=400, detail="amount must be positive")
+    return amount.quantize(Decimal("0.01"))
+
+
+def _require_year_month(year_month: str) -> str:
+    if not month_service.valid_year_month(year_month):
+        raise HTTPException(
+            status_code=404, detail="year_month must be formatted as YYYY-MM"
+        )
+    return year_month
+
+
+def _require_month(db: Session, year_month: str) -> Month:
+    _require_year_month(year_month)
+    month = month_service.get_month(db, year_month)
+    if month is None:
+        raise HTTPException(status_code=404, detail="month has not been created yet")
+    return month
+
+
+def _section_views(month: Month) -> list[month_service.MonthSectionView]:
+    return [
+        month_service.section_view(month, s, SECTION_LABELS[s]) for s in Section
+    ]
+
+
+def _render_section(
+    request: Request, month: Month, section: Section
+) -> HTMLResponse:
+    view = month_service.section_view(month, section, SECTION_LABELS[section])
+    return templates.TemplateResponse(
+        request,
+        "partials/month_section.html",
+        {"month": month, "section": view},
+    )
+
+
+def _render_target(request: Request, db: Session, month: Month) -> HTMLResponse:
+    target = month_service.get_month_target(db, month)
+    debt_minimums = [e for e in month.entries if e.section == Section.debt_minimum]
+    debt_minimums.sort(key=lambda e: e.id)
+    return templates.TemplateResponse(
+        request,
+        "partials/month_target.html",
+        {"month": month, "target": target, "debt_minimums": debt_minimums},
+    )
+
+
+@router.get("", response_class=HTMLResponse)
+def view_month(
+    year_month: str,
+    request: Request,
+    db: Session = Depends(get_session),
+) -> HTMLResponse:
+    _require_year_month(year_month)
+    month = month_service.get_month(db, year_month)
+    prev_ym = month_service.shift_year_month(year_month, -1)
+    next_ym = month_service.shift_year_month(year_month, 1)
+    all_months = month_service.list_months(db)
+    if month is None:
+        return templates.TemplateResponse(
+            request,
+            "month_create.html",
+            {
+                "year_month": year_month,
+                "prev_year_month": prev_ym,
+                "next_year_month": next_ym,
+                "all_months": all_months,
+            },
+        )
+    return templates.TemplateResponse(
+        request,
+        "month.html",
+        {
+            "month": month,
+            "year_month": year_month,
+            "prev_year_month": prev_ym,
+            "next_year_month": next_ym,
+            "all_months": all_months,
+            "sections": _section_views(month),
+            "target": month_service.get_month_target(db, month),
+            "debt_minimums": sorted(
+                (e for e in month.entries if e.section == Section.debt_minimum),
+                key=lambda e: e.id,
+            ),
+        },
+    )
+
+
+@router.post("/create")
+def create_month(
+    year_month: str,
+    db: Session = Depends(get_session),
+) -> Response:
+    _require_year_month(year_month)
+    try:
+        month_service.create_month(db, year_month)
+    except ValueError as exc:
+        raise HTTPException(status_code=400, detail=str(exc)) from exc
+    return Response(
+        status_code=204,
+        headers={"HX-Redirect": f"/month/{year_month}"},
+    )
+
+
+@router.post("/sections/{section}/entries", response_class=HTMLResponse)
+def add_month_entry(
+    year_month: str,
+    section: Section,
+    request: Request,
+    name: str = Form(...),
+    planned: str = Form(...),
+    db: Session = Depends(get_session),
+) -> HTMLResponse:
+    month = _require_month(db, year_month)
+    clean_name = name.strip()
+    if not clean_name:
+        raise HTTPException(status_code=400, detail="name is required")
+    month_service.add_month_entry(
+        db, month, section, clean_name, _parse_amount(planned)
+    )
+    db.refresh(month)
+    return _render_section(request, month, section)
+
+
+@router.delete("/entries/{entry_id}", response_class=HTMLResponse)
+def delete_month_entry(
+    year_month: str,
+    entry_id: int,
+    request: Request,
+    db: Session = Depends(get_session),
+) -> HTMLResponse:
+    month = _require_month(db, year_month)
+    section = month_service.delete_month_entry(db, month, entry_id)
+    if section is None:
+        raise HTTPException(status_code=404, detail="entry not found")
+    db.refresh(month)
+    response = _render_section(request, month, section)
+    if section == Section.debt_minimum:
+        target_html = _render_target(request, db, month).body.decode()
+        response = HTMLResponse(response.body.decode() + target_html)
+    return response
+
+
+@router.post("/entries/{entry_id}", response_class=HTMLResponse)
+def update_month_entry(
+    year_month: str,
+    entry_id: int,
+    request: Request,
+    name: str | None = Form(None),
+    planned: str | None = Form(None),
+    applied: str | None = Form(None),
+    db: Session = Depends(get_session),
+) -> HTMLResponse:
+    month = _require_month(db, year_month)
+    clean_name: str | None = None
+    if name is not None:
+        clean_name = name.strip()
+        if not clean_name:
+            raise HTTPException(status_code=400, detail="name must not be empty")
+    parsed_planned = _parse_amount(planned) if planned is not None else None
+    parsed_applied = _parse_amount(applied) if applied is not None else None
+    updated = month_service.update_month_entry(
+        db,
+        month,
+        entry_id,
+        name=clean_name,
+        planned=parsed_planned,
+        applied=parsed_applied,
+    )
+    if updated is None:
+        raise HTTPException(status_code=404, detail="entry not found")
+    db.refresh(month)
+    return _render_section(request, month, updated.section)
+
+
+@router.post("/target", response_class=HTMLResponse)
+def update_month_target(
+    year_month: str,
+    request: Request,
+    month_entry_id: str = Form(""),
+    db: Session = Depends(get_session),
+) -> HTMLResponse:
+    month = _require_month(db, year_month)
+    raw = month_entry_id.strip()
+    target_id: int | None
+    if raw == "":
+        target_id = None
+    else:
+        try:
+            target_id = int(raw)
+        except ValueError as exc:
+            raise HTTPException(
+                status_code=400, detail="month_entry_id must be an integer"
+            ) from exc
+    try:
+        month_service.set_month_target(db, month, target_id)
+    except ValueError as exc:
+        raise HTTPException(status_code=400, detail=str(exc)) from exc
+    return _render_target(request, db, month)

src/quartermaster/static/app.css

@@ -156,3 +156,124 @@ button[type=submit] {
 .target-section .section-header {
   border-bottom-style: dashed;
 }
+
+/* Monthly view ----------------------------------------------------------- */
+
+.month-nav {
+  display: flex;
+  align-items: center;
+  gap: 0.75rem;
+  padding: 0.5rem 0;
+  margin-top: 1rem;
+  flex-wrap: wrap;
+}
+
+.month-nav .month-label {
+  font-size: 1.2rem;
+  font-weight: 600;
+  padding: 0 0.5rem;
+}
+
+.month-nav .nav-link {
+  color: var(--accent);
+  text-decoration: none;
+  padding: 0.25rem 0.5rem;
+  border: 1px solid var(--rule);
+  border-radius: 3px;
+  background: #fff;
+  font-size: 0.9rem;
+}
+
+.month-nav .nav-link:hover {
+  background: #f0ece0;
+}
+
+.month-nav .spacer {
+  flex: 1;
+}
+
+.month-nav .month-picker {
+  padding: 0.3rem 0.5rem;
+  font-size: 0.9rem;
+}
+
+.total .divider {
+  color: var(--muted);
+  margin: 0 0.25rem;
+  font-weight: 400;
+}
+
+.total .planned {
+  color: var(--muted);
+}
+
+table.month-entries thead th {
+  text-align: left;
+  padding: 0.25rem 0.5rem;
+  font-size: 0.8rem;
+  color: var(--muted);
+  font-weight: 500;
+  border-bottom: 1px solid var(--rule);
+}
+
+table.month-entries th.col-planned,
+table.month-entries th.col-applied {
+  text-align: right;
+  width: 8rem;
+}
+
+table.month-entries th.col-actions {
+  width: 2.25rem;
+}
+
+table.month-entries td.entry-name input[name="name"] {
+  width: 100%;
+  min-width: 8rem;
+}
+
+table.month-entries td.entry-amount input {
+  width: 100%;
+  text-align: right;
+}
+
+tr.entry.state-edited {
+  background: #fff7e0;
+}
+
+tr.entry.state-new_in_month {
+  background: #e8f2ff;
+}
+
+.tag {
+  display: inline-block;
+  margin-left: 0.4rem;
+  padding: 0.05rem 0.4rem;
+  border-radius: 3px;
+  font-size: 0.72rem;
+  font-weight: 600;
+  text-transform: lowercase;
+  letter-spacing: 0.02em;
+}
+
+.tag-edited {
+  background: #f4d07a;
+  color: #5a3f00;
+}
+
+.tag-new {
+  background: #9cc5ef;
+  color: #123a66;
+}
+
+.month-add-form {
+  grid-template-columns: 1fr 9rem auto;
+}
+
+.month-missing-body {
+  padding: 0.75rem 0.5rem;
+  color: var(--muted);
+}
+
+.month-create-form {
+  padding: 0.25rem 0.5rem 0.75rem;
+}

src/quartermaster/templates/index.html

@@ -1,6 +1,23 @@
 {% extends "base.html" %}
 {% block content %}
 <div class="budget">
+  <nav class="month-nav budget-nav">
+    <span class="month-label">Budget configuration</span>
+    <span class="spacer"></span>
+    <a class="nav-link" href="/month/{{ current_year_month }}">This month ({{ current_year_month }})</a>
+    {% if all_months %}
+      <select
+        class="month-picker"
+        onchange="if(this.value){window.location=this.value}"
+        aria-label="Jump to month"
+      >
+        <option value="">Jump to...</option>
+        {% for m in all_months %}
+          <option value="/month/{{ m.year_month }}">{{ m.year_month }}</option>
+        {% endfor %}
+      </select>
+    {% endif %}
+  </nav>
   {% for section in sections %}
     {% if section.section.value == 'debt_minimum' %}
       {% include "partials/section.html" %}

src/quartermaster/templates/month.html

@@ -0,0 +1,14 @@
+{% extends "base.html" %}
+{% block content %}
+<div class="budget">
+  {% include "partials/month_nav.html" %}
+  {% for section in sections %}
+    {% if section.section.value == 'debt_minimum' %}
+      {% include "partials/month_section.html" %}
+      {% include "partials/month_target.html" %}
+    {% else %}
+      {% include "partials/month_section.html" %}
+    {% endif %}
+  {% endfor %}
+</div>
+{% endblock %}

src/quartermaster/templates/month_create.html

@@ -0,0 +1,23 @@
+{% extends "base.html" %}
+{% block content %}
+<div class="budget">
+  {% include "partials/month_nav.html" %}
+  <section class="section month-missing">
+    <div class="section-header">
+      <h2>No snapshot yet</h2>
+    </div>
+    <p class="month-missing-body">
+      This month has not been created. Creating it will snapshot the current
+      budget: every entry, its planned amount, and the current Primary Debt
+      Target. Applied amounts start at $0.00.
+    </p>
+    <form
+      hx-post="/month/{{ year_month }}/create"
+      hx-swap="none"
+      class="month-create-form"
+    >
+      <button type="submit">Create {{ year_month }}</button>
+    </form>
+  </section>
+</div>
+{% endblock %}

src/quartermaster/templates/partials/month_nav.html

@@ -0,0 +1,22 @@
+<nav class="month-nav">
+  <a class="nav-link" href="/month/{{ prev_year_month }}" aria-label="Previous month">&larr; {{ prev_year_month }}</a>
+  <span class="month-label">{{ year_month }}</span>
+  <a class="nav-link" href="/month/{{ next_year_month }}" aria-label="Next month">{{ next_year_month }} &rarr;</a>
+  <span class="spacer"></span>
+  {% if all_months %}
+    <select
+      class="month-picker"
+      onchange="if(this.value){window.location=this.value}"
+      aria-label="Jump to month"
+    >
+      <option value="">Jump to...</option>
+      {% for m in all_months %}
+        <option
+          value="/month/{{ m.year_month }}"
+          {% if m.year_month == year_month %}selected{% endif %}
+        >{{ m.year_month }}</option>
+      {% endfor %}
+    </select>
+  {% endif %}
+  <a class="nav-link" href="/">Budget config</a>
+</nav>

src/quartermaster/templates/partials/month_section.html

@@ -0,0 +1,91 @@
+<section class="section" id="section-{{ section.section.value }}">
+  <div class="section-header">
+    <h2>{{ section.label }}</h2>
+    <span class="total" data-testid="total-{{ section.section.value }}">
+      <span class="applied">${{ '%.2f' | format(section.total_applied) }}</span>
+      <span class="divider">/</span>
+      <span class="planned">${{ '%.2f' | format(section.total_planned) }}</span>
+    </span>
+  </div>
+  <table class="entries month-entries">
+    <thead>
+      <tr>
+        <th class="col-name">Name</th>
+        <th class="col-planned">Planned</th>
+        <th class="col-applied">Applied</th>
+        <th class="col-actions"></th>
+      </tr>
+    </thead>
+    <tbody>
+      {% for row in section.rows %}
+        <tr class="entry state-{{ row.state.value }}" data-entry-id="{{ row.entry.id }}">
+          <td class="entry-name">
+            <input
+              type="text"
+              name="name"
+              value="{{ row.entry.name }}"
+              hx-post="/month/{{ month.year_month }}/entries/{{ row.entry.id }}"
+              hx-trigger="change"
+              hx-target="#section-{{ section.section.value }}"
+              hx-swap="outerHTML"
+            >
+            {% if row.state.value == 'edited' %}
+              <span class="tag tag-edited">modified</span>
+            {% elif row.state.value == 'new_in_month' %}
+              <span class="tag tag-new">new this month</span>
+            {% endif %}
+          </td>
+          <td class="entry-amount">
+            <input
+              type="number" step="0.01" min="0"
+              name="planned"
+              value="{{ '%.2f' | format(row.entry.planned) }}"
+              hx-post="/month/{{ month.year_month }}/entries/{{ row.entry.id }}"
+              hx-trigger="change"
+              hx-target="#section-{{ section.section.value }}"
+              hx-swap="outerHTML"
+            >
+          </td>
+          <td class="entry-amount">
+            <input
+              type="number" step="0.01" min="0"
+              name="applied"
+              value="{{ '%.2f' | format(row.entry.applied) }}"
+              hx-post="/month/{{ month.year_month }}/entries/{{ row.entry.id }}"
+              hx-trigger="change"
+              hx-target="#section-{{ section.section.value }}"
+              hx-swap="outerHTML"
+            >
+          </td>
+          <td class="entry-actions">
+            <button
+              class="delete"
+              type="button"
+              hx-delete="/month/{{ month.year_month }}/entries/{{ row.entry.id }}"
+              hx-target="#section-{{ section.section.value }}"
+              hx-swap="outerHTML"
+              aria-label="Delete {{ row.entry.name }}"
+            >&times;</button>
+          </td>
+        </tr>
+      {% else %}
+        <tr class="empty"><td colspan="4">No entries.</td></tr>
+      {% endfor %}
+      <tr class="add-row">
+        <td colspan="4">
+          <form
+            class="add-form month-add-form"
+            hx-post="/month/{{ month.year_month }}/sections/{{ section.section.value }}/entries"
+            hx-target="#section-{{ section.section.value }}"
+            hx-swap="outerHTML"
+            hx-on::after-request="if(event.detail.successful) this.reset()"
+          >
+            <input type="text" name="name" placeholder="Name" required>
+            <input type="number" name="planned" step="0.01" min="0" placeholder="Planned" required>
+            <button type="submit">Add</button>
+          </form>
+        </td>
+      </tr>
+    </tbody>
+  </table>
+</section>

src/quartermaster/templates/partials/month_target.html

@@ -0,0 +1,46 @@
+<section class="section target-section" id="section-debt_target" hx-swap-oob="outerHTML">
+  <div class="section-header">
+    <h2>Primary Debt Target</h2>
+    {% if target.entry %}
+      <span class="total">
+        <span class="applied">${{ '%.2f' | format(target.entry.applied) }}</span>
+        <span class="divider">/</span>
+        <span class="planned">${{ '%.2f' | format(target.entry.planned) }}</span>
+      </span>
+    {% else %}
+      <span class="total empty">$0.00</span>
+    {% endif %}
+  </div>
+  <table class="entries">
+    <tbody>
+      <tr class="entry">
+        <td class="entry-name">
+          {% if target.entry %}{{ target.entry.name }}{% else %}<span class="muted">No target selected.</span>{% endif %}
+        </td>
+        <td class="entry-amount"></td>
+        <td class="entry-actions"></td>
+      </tr>
+      <tr class="add-row">
+        <td colspan="3">
+          <form
+            class="target-form"
+            hx-post="/month/{{ month.year_month }}/target"
+            hx-target="#section-debt_target"
+            hx-swap="outerHTML"
+          >
+            <select name="month_entry_id">
+              <option value="">(none)</option>
+              {% for dm in debt_minimums %}
+                <option
+                  value="{{ dm.id }}"
+                  {% if target.month_entry_id == dm.id %}selected{% endif %}
+                >{{ dm.name }}: ${{ '%.2f' | format(dm.planned) }}</option>
+              {% endfor %}
+            </select>
+            <button type="submit">Set</button>
+          </form>
+        </td>
+      </tr>
+    </tbody>
+  </table>
+</section>

tests/test_month_routes.py

@@ -0,0 +1,139 @@
+from __future__ import annotations
+
+
+def _seed_budget_via_api(client):
+    client.post(
+        "/sections/income/entries",
+        data={"name": "Paycheck", "amount": "2500.00"},
+    )
+    client.post(
+        "/sections/fixed_bill/entries",
+        data={"name": "Rent", "amount": "1200.00"},
+    )
+    client.post(
+        "/sections/debt_minimum/entries",
+        data={"name": "Card A", "amount": "40.00"},
+    )
+
+
+def test_missing_month_renders_create_flow(client):
+    response = client.get("/month/2026-04")
+    assert response.status_code == 200
+    assert "No snapshot yet" in response.text
+    assert 'hx-post="/month/2026-04/create"' in response.text
+
+
+def test_create_month_redirects_via_htmx(client):
+    _seed_budget_via_api(client)
+    response = client.post("/month/2026-04/create")
+    assert response.status_code == 204
+    assert response.headers.get("hx-redirect") == "/month/2026-04"
+
+
+def test_created_month_renders_snapshot(client):
+    _seed_budget_via_api(client)
+    client.post("/month/2026-04/create")
+    response = client.get("/month/2026-04")
+    assert response.status_code == 200
+    assert "Paycheck" in response.text
+    assert "Rent" in response.text
+    assert "Card A" in response.text
+    # totals: applied / planned
+    assert "$2500.00" in response.text
+    assert "$1200.00" in response.text
+
+
+def test_applied_update_returns_section_partial(client):
+    _seed_budget_via_api(client)
+    client.post("/month/2026-04/create")
+    # fetch month page, identify rent month_entry id = 2 (after income id 1)
+    response = client.post(
+        "/month/2026-04/entries/2", data={"applied": "1200.00"}
+    )
+    assert response.status_code == 200
+    # rent is in fixed_bill section; total_applied should now include 1200
+    assert 'id="section-fixed_bill"' in response.text
+    # two $1200.00 occurrences: total_applied and planned (same amount)
+    assert response.text.count("$1200.00") >= 2
+
+
+def test_name_edit_flips_to_modified(client):
+    _seed_budget_via_api(client)
+    client.post("/month/2026-04/create")
+    response = client.post(
+        "/month/2026-04/entries/2", data={"name": "Rent (April)"}
+    )
+    assert response.status_code == 200
+    assert "state-edited" in response.text
+    assert 'class="tag tag-edited">modified' in response.text
+
+
+def test_add_new_entry_within_month_is_marked_new(client):
+    _seed_budget_via_api(client)
+    client.post("/month/2026-04/create")
+    response = client.post(
+        "/month/2026-04/sections/other/entries",
+        data={"name": "Gift", "planned": "50.00"},
+    )
+    assert response.status_code == 200
+    assert "state-new_in_month" in response.text
+    assert "new this month" in response.text
+
+
+def test_delete_month_entry(client):
+    _seed_budget_via_api(client)
+    client.post("/month/2026-04/create")
+    response = client.delete("/month/2026-04/entries/2")
+    assert response.status_code == 200
+    assert "Rent" not in response.text
+
+
+def test_delete_month_debt_minimum_updates_target(client):
+    _seed_budget_via_api(client)
+    client.post("/month/2026-04/create")
+    # Card A is entry id 3 in the month snapshot
+    response = client.delete("/month/2026-04/entries/3")
+    assert response.status_code == 200
+    assert "section-debt_target" in response.text
+    assert "No target selected" in response.text
+
+
+def test_set_month_target(client):
+    _seed_budget_via_api(client)
+    client.post(
+        "/sections/debt_minimum/entries",
+        data={"name": "Card B", "amount": "60.00"},
+    )
+    client.post("/month/2026-04/create")
+    # Card B is month_entry id 4
+    response = client.post(
+        "/month/2026-04/target", data={"month_entry_id": "4"}
+    )
+    assert response.status_code == 200
+    assert "Card B" in response.text
+
+
+def test_month_target_isolated_between_months(client):
+    _seed_budget_via_api(client)
+    client.post(
+        "/sections/debt_minimum/entries",
+        data={"name": "Card B", "amount": "60.00"},
+    )
+    client.post("/month/2026-04/create")
+    client.post("/month/2026-05/create")
+    # Change April target to Card B (month_entry id 4 within April)
+    client.post("/month/2026-04/target", data={"month_entry_id": "4"})
+    may_page = client.get("/month/2026-05")
+    # May still points at Card A (copied from budget)
+    assert "Card A" in may_page.text
+
+
+def test_malformed_year_month_returns_404(client):
+    response = client.get("/month/2026-13")
+    assert response.status_code == 404
+
+
+def test_budget_page_shows_month_nav(client):
+    response = client.get("/")
+    assert response.status_code == 200
+    assert "This month" in response.text

tests/test_month_service.py

@@ -0,0 +1,148 @@
+from __future__ import annotations
+
+from decimal import Decimal
+
+import pytest
+
+from quartermaster import month_service, service
+from quartermaster.models import Section
+
+
+def _seed_budget(db):
+    income = service.add_entry(db, Section.income, "Paycheck", Decimal("2500.00"))
+    rent = service.add_entry(
+        db, Section.fixed_bill, "Rent", Decimal("1200.00")
+    )
+    card_a = service.add_entry(
+        db, Section.debt_minimum, "Card A", Decimal("40.00")
+    )
+    card_b = service.add_entry(
+        db, Section.debt_minimum, "Card B", Decimal("60.00")
+    )
+    service.set_debt_target(db, card_a.id)
+    return income, rent, card_a, card_b
+
+
+def test_valid_year_month():
+    assert month_service.valid_year_month("2026-04")
+    assert month_service.valid_year_month("2026-12")
+    assert not month_service.valid_year_month("2026-00")
+    assert not month_service.valid_year_month("2026-13")
+    assert not month_service.valid_year_month("26-04")
+    assert not month_service.valid_year_month("2026/04")
+
+
+def test_shift_year_month_rolls_years():
+    assert month_service.shift_year_month("2026-12", 1) == "2027-01"
+    assert month_service.shift_year_month("2026-01", -1) == "2025-12"
+    assert month_service.shift_year_month("2026-04", 0) == "2026-04"
+    assert month_service.shift_year_month("2026-04", 12) == "2027-04"
+
+
+def test_create_month_snapshots_budget(db):
+    income, rent, card_a, card_b = _seed_budget(db)
+    month = month_service.create_month(db, "2026-04")
+    assert month.year_month == "2026-04"
+    assert len(month.entries) == 4
+    by_source = {e.source_entry_id: e for e in month.entries}
+    assert by_source[income.id].planned == Decimal("2500.00")
+    assert by_source[income.id].origin_name == "Paycheck"
+    assert by_source[income.id].origin_planned == Decimal("2500.00")
+    assert by_source[income.id].applied == Decimal("0.00")
+    assert by_source[rent.id].section == Section.fixed_bill
+    assert month.target is not None
+    assert month.target.entry is not None
+    assert month.target.entry.source_entry_id == card_a.id
+
+
+def test_create_month_idempotent(db):
+    _seed_budget(db)
+    first = month_service.create_month(db, "2026-04")
+    second = month_service.create_month(db, "2026-04")
+    assert first.id == second.id
+
+
+def test_create_month_rejects_bad_year_month(db):
+    with pytest.raises(ValueError):
+        month_service.create_month(db, "2026-00")
+
+
+def test_deviation_state_reflects_edits(db):
+    _seed_budget(db)
+    month = month_service.create_month(db, "2026-04")
+    rent = next(e for e in month.entries if e.origin_name == "Rent")
+    assert month_service.deviation_state(rent) == month_service.DeviationState.unchanged
+
+    month_service.update_month_entry(
+        db, month, rent.id, planned=Decimal("1300.00")
+    )
+    rent = next(e for e in month.entries if e.id == rent.id)
+    assert month_service.deviation_state(rent) == month_service.DeviationState.edited
+
+
+def test_added_row_is_new_in_month(db):
+    _seed_budget(db)
+    month = month_service.create_month(db, "2026-04")
+    extra = month_service.add_month_entry(
+        db, month, Section.other, "Surprise", Decimal("20.00")
+    )
+    assert extra.origin_name is None
+    assert month_service.deviation_state(extra) == (
+        month_service.DeviationState.new_in_month
+    )
+
+
+def test_applied_update_changes_totals(db):
+    _seed_budget(db)
+    month = month_service.create_month(db, "2026-04")
+    rent = next(e for e in month.entries if e.origin_name == "Rent")
+    month_service.update_month_entry(
+        db, month, rent.id, applied=Decimal("1200.00")
+    )
+    view = month_service.section_view(month, Section.fixed_bill, "Fixed")
+    assert view.total_applied == Decimal("1200.00")
+    assert view.total_planned == Decimal("1200.00")
+
+
+def test_per_month_target_is_isolated(db):
+    _seed_budget(db)
+    april = month_service.create_month(db, "2026-04")
+    may = month_service.create_month(db, "2026-05")
+    april_b = next(
+        e for e in april.entries
+        if e.section == Section.debt_minimum and e.origin_name == "Card B"
+    )
+    month_service.set_month_target(db, april, april_b.id)
+    april_target = month_service.get_month_target(db, april)
+    may_target = month_service.get_month_target(db, may)
+    assert april_target.entry is not None
+    assert april_target.entry.origin_name == "Card B"
+    assert may_target.entry is not None
+    assert may_target.entry.origin_name == "Card A"
+
+
+def test_delete_month_entry_returns_section(db):
+    _seed_budget(db)
+    month = month_service.create_month(db, "2026-04")
+    rent = next(e for e in month.entries if e.origin_name == "Rent")
+    result = month_service.delete_month_entry(db, month, rent.id)
+    assert result == Section.fixed_bill
+    db.refresh(month)
+    remaining = [
+        e for e in month.entries if e.section == Section.fixed_bill
+    ]
+    assert remaining == []
+
+
+def test_budget_entry_deleted_after_snapshot_leaves_month_intact(db):
+    _seed_budget(db)
+    month = month_service.create_month(db, "2026-04")
+    # remove budget-side rent; month-side rent should still render unchanged
+    budget_rent = service.list_entries(db, Section.fixed_bill)[0]
+    service.delete_entry(db, budget_rent.id)
+    db.expire_all()
+    month_rent = next(e for e in month.entries if e.origin_name == "Rent")
+    assert month_rent.source_entry_id is None
+    assert month_service.deviation_state(month_rent) == (
+        month_service.DeviationState.unchanged
+    )