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/models/models.py

@@ -333,3 +333,28 @@ class PensionSnapshot(PensionSnapshotBase, table=True):
 class PensionSnapshotRead(PensionSnapshotBase):
     id: int
     created_at: datetime
+
+
+# --- Balance Override ---
+
+
+class BalanceOverride(SQLModel, table=True):
+    __table_args__ = (UniqueConstraint("year", "month"),)
+    id: Optional[int] = Field(default=None, primary_key=True)
+    year: int
+    month: int
+    override_balance: float
+    created_at: datetime = Field(default_factory=datetime.utcnow)
+    updated_at: datetime = Field(default_factory=datetime.utcnow)
+
+
+class BalanceOverrideCreate(SQLModel):
+    override_balance: float
+
+
+class BalanceOverrideRead(SQLModel):
+    id: int
+    year: int
+    month: int
+    override_balance: float
+    updated_at: datetime