feat(zero): compute zero-amount for budget and month

budget_zero sums income minus every non-income section on the budget
config. month_zero returns both Planned and Applied versions over a
month's entries. zero_tone classifies a value as zero / positive /
negative so templates can pick a colour.

Refs #7

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

src/quartermaster/month_service.py

@@ -42,6 +42,12 @@ class MonthSectionView:
     total_applied: Decimal
 
 
+@dataclass(frozen=True)
+class ZeroAmounts:
+    planned: Decimal
+    applied: Decimal
+
+
 def valid_year_month(year_month: str) -> bool:
     return bool(YEAR_MONTH_RE.match(year_month))
 
@@ -124,6 +130,24 @@ def _rows(entries: list[MonthEntry]) -> list[MonthRow]:
     return [MonthRow(entry=e, state=deviation_state(e)) for e in entries]
 
 
+def month_zero(month: Month) -> ZeroAmounts:
+    income_planned = Decimal("0")
+    income_applied = Decimal("0")
+    other_planned = Decimal("0")
+    other_applied = Decimal("0")
+    for entry in month.entries:
+        if entry.section == Section.income:
+            income_planned += entry.planned
+            income_applied += entry.applied
+        else:
+            other_planned += entry.planned
+            other_applied += entry.applied
+    return ZeroAmounts(
+        planned=(income_planned - other_planned).quantize(Decimal("0.01")),
+        applied=(income_applied - other_applied).quantize(Decimal("0.01")),
+    )
+
+
 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)

src/quartermaster/service.py

@@ -9,6 +9,12 @@ from sqlalchemy.orm import Session
 from quartermaster.models import DebtTarget, Entry, Section
 
 
+def zero_tone(value: Decimal) -> str:
+    if value == 0:
+        return "zero"
+    return "positive" if value > 0 else "negative"
+
+
 @dataclass(frozen=True)
 class SectionView:
     section: Section
@@ -26,6 +32,18 @@ def section_total(entries: list[Entry]) -> Decimal:
     return sum((e.amount for e in entries), Decimal("0"))
 
 
+def budget_zero(db: Session) -> Decimal:
+    stmt = select(Entry)
+    total_income = Decimal("0")
+    total_non_income = Decimal("0")
+    for entry in db.scalars(stmt):
+        if entry.section == Section.income:
+            total_income += entry.amount
+        else:
+            total_non_income += entry.amount
+    return (total_income - total_non_income).quantize(Decimal("0.01"))
+
+
 def add_entry(db: Session, section: Section, name: str, amount: Decimal) -> Entry:
     entry = Entry(section=section, name=name.strip(), amount=amount)
     db.add(entry)