Add cumulative balance tracking with editable overrides

- New BalanceOverride table for manual balance adjustments per month
- Cumulative balance computation with cross-year carryover
- Three new columns: Acum. Anterior, Neto Mes, Balance Acum.
- Inline editing on Balance Acum. cell (pencil icon for overrides)
- Year navigation clamped to 2026–2030, fresh start at March 2026
- PUT/DELETE /budget/balance-override/{year}/{month} endpoints

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/app/services/budget_projection.py

@@ -4,6 +4,7 @@ from datetime import datetime
 from sqlmodel import Session, func, select
 
 from app.models.models import (
+    BalanceOverride,
     RecurringFrequency,
     RecurringItem,
     RecurringItemType,
@@ -12,6 +13,12 @@ from app.models.models import (
     TransactionType,
 )
 
+MIN_YEAR = 2026
+MAX_YEAR = 2030
+# Fresh start: months before this are zeroed out
+FRESH_START_YEAR = 2026
+FRESH_START_MONTH = 3
+
 
 def get_effective_amount(item: RecurringItem, month: int, year: int) -> float | None:
     """Return the effective amount for a recurring item in a given month, or None if inactive."""
@@ -255,3 +262,80 @@ def compute_monthly_projection(
         "savings_items": savings_items,
         "actuals_by_source": list(actuals_by_source.values()),
     }
+
+
+def _get_december_cumulative(session: Session, year: int) -> float:
+    """Get the cumulative balance for December of a given year."""
+    # Check for an override first
+    override = session.exec(
+        select(BalanceOverride).where(
+            BalanceOverride.year == year, BalanceOverride.month == 12
+        )
+    ).first()
+    if override:
+        return 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}
+
+    cumulative = 0.0
+    if year > FRESH_START_YEAR:
+        cumulative = _get_december_cumulative(session, year - 1)
+
+    for m in range(1, 13):
+        if year == FRESH_START_YEAR and m < FRESH_START_MONTH:
+            continue
+        data = compute_monthly_projection(session, year, m)
+        cumulative += data["net_balance"]
+        if m in override_map:
+            cumulative = override_map[m]
+
+    return cumulative
+
+
+def compute_yearly_projection_with_cumulative(
+    session: Session, year: int
+) -> list[dict]:
+    """Compute all 12 months with cumulative balance tracking."""
+    overrides = session.exec(
+        select(BalanceOverride).where(BalanceOverride.year == year)
+    ).all()
+    override_map = {o.month: o.override_balance for o in overrides}
+
+    # Determine January carryover
+    if year <= FRESH_START_YEAR:
+        carryover = 0.0
+    else:
+        carryover = _get_december_cumulative(session, year - 1)
+
+    months = []
+    for m in range(1, 13):
+        data = compute_monthly_projection(session, year, m)
+
+        is_before_fresh_start = (
+            year == FRESH_START_YEAR and m < FRESH_START_MONTH
+        )
+
+        if is_before_fresh_start:
+            data["carryover_balance"] = 0.0
+            data["cumulative_balance"] = 0.0
+            data["balance_overridden"] = False
+        else:
+            data["carryover_balance"] = carryover
+            cumulative = carryover + data["net_balance"]
+
+            if m in override_map:
+                cumulative = override_map[m]
+                data["balance_overridden"] = True
+            else:
+                data["balance_overridden"] = False
+
+            data["cumulative_balance"] = cumulative
+            carryover = cumulative
+
+        months.append(data)
+
+    return months