OpenAPI type codegen with CI drift gate (plan 3.4, ARCH-16, FE-14)

Every dict-returning route the SPA consumes now declares a response
model (auth, bulk, sync-status, notifications). api-types.gen.ts is
generated from app.openapi() via openapi-typescript, and the 22 hand-
written read interfaces in api.ts are now aliases onto the generated
schemas — backend schema drift becomes a typecheck failure, and CI
regenerates the file and fails the deploy if it's stale. The aliasing
immediately caught two real drifts: RecurringItemType was missing
SAVINGS, and raw_charges (untyped JSON column) is now explicitly
shaped at the boundary.

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

backend/app/api/v1/endpoints/sync_status.py

@@ -1,4 +1,5 @@
 from fastapi import APIRouter, Depends
+from pydantic import BaseModel
 from sqlmodel import Session
 
 from app.auth import get_current_user
@@ -8,7 +9,22 @@ from app.services.sync_status import compute_sync_status
 router = APIRouter(prefix="/sync-status", tags=["sync-status"])
 
 
-@router.get("/")
+class SyncSourceStatus(BaseModel):
+    key: str
+    label: str
+    description: str
+    last_received: str | None
+    age_days: float | None
+    warn_after_days: float
+    status: str  # 'ok' | 'warning' | 'never'
+
+
+class SyncStatusResponse(BaseModel):
+    sources: list[SyncSourceStatus]
+    warnings: int
+
+
+@router.get("/", response_model=SyncStatusResponse)
 def sync_status(
     session: Session = Depends(get_session),
     _user: str = Depends(get_current_user),