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/transactions.py

@@ -106,7 +106,11 @@ class BulkActionRequest(BaseModel):
     deferred: Optional[bool] = None  # for set_deferred
 
 
-@router.post("/bulk")
+class BulkActionResponse(BaseModel):
+    affected: int
+
+
+@router.post("/bulk", response_model=BulkActionResponse)
 def bulk_action(
     req: BulkActionRequest,
     session: Session = Depends(get_session),