Migrate all 29 money columns from float to NUMERIC/Decimal (BE-02)

Models use Money = Annotated[Decimal, PlainSerializer(float)] so storage
is exact NUMERIC(15,2) (rates 15,6) while JSON keeps emitting plain
numbers — the frontend contract is unchanged, pinned by
test_models_serialization.py. Service layers coerce to float at their
boundaries (projection math, rate multipliers, agent tool output, the
savings constants become Decimal) so no naive Decimal/float mixing can
raise. Migration 7f567c8bafdb applied to dev: per-row values and the
table sum byte-identical before/after; endpoints verified live on the
new schema. 59/59 tests green.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend/app/services/budget_projection.py

@@ -31,7 +31,7 @@ def get_effective_amount(item: RecurringItem, month: int, year: int) -> float |
     if freq == RecurringFrequency.MONTHLY:
         if item.override_amounts and str(month) in item.override_amounts:
             return float(item.override_amounts[str(month)])
-        return item.amount
+        return float(item.amount)
 
     if freq == RecurringFrequency.WEEKLY:
         # Count occurrences of the weekday in this month
@@ -39,14 +39,14 @@ def get_effective_amount(item: RecurringItem, month: int, year: int) -> float |
         weekday = item.day_of_month if item.day_of_month is not None else 0
         cal = calendar.monthcalendar(year, month)
         count = sum(1 for week in cal if week[weekday] != 0)
-        return item.amount * count
+        return float(item.amount) * count
 
     if freq == RecurringFrequency.QUARTERLY:
         # Active in months 3, 6, 9, 12 by default
         if month % 3 == 0:
             if item.override_amounts and str(month) in item.override_amounts:
                 return float(item.override_amounts[str(month)])
-            return item.amount
+            return float(item.amount)
         return None
 
     if freq == RecurringFrequency.BIANNUAL:
@@ -54,12 +54,12 @@ def get_effective_amount(item: RecurringItem, month: int, year: int) -> float |
         base = item.month_of_year or 1
         second = base + 6 if base <= 6 else base - 6
         if month in (base, second):
-            return item.amount
+            return float(item.amount)
         return None
 
     if freq == RecurringFrequency.YEARLY:
         if month == (item.month_of_year or 12):
-            return item.amount
+            return float(item.amount)
         return None
 
     return None
@@ -561,13 +561,13 @@ def _get_december_cumulative(session: Session, year: int) -> float:
         )
     ).first()
     if override:
-        return override.override_balance
+        return float(override.override_balance)
 
     # Compute the full year to get December's cumulative
     overrides = session.exec(
         select(BalanceOverride).where(BalanceOverride.year == year)
     ).all()
-    override_map = {o.month: o.override_balance for o in overrides}
+    override_map = {o.month: float(o.override_balance) for o in overrides}
 
     cumulative = 0.0
     if year > FRESH_START_YEAR:
@@ -591,7 +591,7 @@ def compute_yearly_projection_with_cumulative(
     overrides = session.exec(
         select(BalanceOverride).where(BalanceOverride.year == year)
     ).all()
-    override_map = {o.month: o.override_balance for o in overrides}
+    override_map = {o.month: float(o.override_balance) for o in overrides}
 
     # Determine January carryover
     if year <= FRESH_START_YEAR: