feat(lifecycle): activate, close, reopen transitions with validation

activate_month moves Planning to Active and stamps activated_at.
close_month moves Active to Closed only when applied zero equals
exactly $0.00; otherwise raises MonthLifecycleError with a message
naming the current balance. reopen_month moves Closed back to
Active and nulls closed_at. ensure_editable is the guard mutation
routes call before any write. No automatic sweep: filling the
target row is the user's job via editing applied amounts.

Refs #15

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

src/quartermaster/month_service.py

@@ -2,7 +2,7 @@ from __future__ import annotations
 
 import re
 from dataclasses import dataclass
-from datetime import date
+from datetime import date, datetime, timezone
 from decimal import Decimal
 from enum import Enum
 
@@ -23,6 +23,7 @@ from quartermaster.models import (
     Month,
     MonthDebtTarget,
     MonthEntry,
+    MonthState,
     Section,
 )
 
@@ -306,3 +307,54 @@ def set_month_target(
     db.commit()
     db.refresh(target)
     return target
+
+
+class MonthLifecycleError(Exception):
+    """Raised when a state transition or edit is rejected."""
+
+
+def activate_month(db: Session, month: Month) -> Month:
+    if month.state != MonthState.planning:
+        raise MonthLifecycleError(
+            f"cannot activate a {month.state.value} month"
+        )
+    month.state = MonthState.active
+    month.activated_at = datetime.now(timezone.utc)
+    db.commit()
+    db.refresh(month)
+    return month
+
+
+def close_month(db: Session, month: Month) -> Month:
+    if month.state != MonthState.active:
+        raise MonthLifecycleError(
+            f"cannot close a {month.state.value} month"
+        )
+    zero = month_zero(month)
+    if zero.applied != Decimal("0.00"):
+        raise MonthLifecycleError(
+            "applied balance must equal $0.00 before closing; "
+            f"currently ${zero.applied}"
+        )
+    month.state = MonthState.closed
+    month.closed_at = datetime.now(timezone.utc)
+    db.commit()
+    db.refresh(month)
+    return month
+
+
+def reopen_month(db: Session, month: Month) -> Month:
+    if month.state != MonthState.closed:
+        raise MonthLifecycleError(
+            f"cannot reopen a {month.state.value} month"
+        )
+    month.state = MonthState.active
+    month.closed_at = None
+    db.commit()
+    db.refresh(month)
+    return month
+
+
+def ensure_editable(month: Month) -> None:
+    if month.state == MonthState.closed:
+        raise MonthLifecycleError("month is closed; reopen to edit")