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/notifications.py

@@ -1,6 +1,7 @@
 import json
 import logging
 
+from pydantic import BaseModel
 from fastapi import APIRouter, Depends, HTTPException
 from pywebpush import WebPushException, webpush
 from sqlmodel import Session, select
@@ -15,14 +16,22 @@ logger = logging.getLogger(__name__)
 router = APIRouter(prefix="/notifications", tags=["notifications"])
 
 
-@router.get("/vapid-public-key")
+class VapidPublicKeyResponse(BaseModel):
+    publicKey: str
+
+
+class SubscriptionStatusResponse(BaseModel):
+    status: str
+
+
+@router.get("/vapid-public-key", response_model=VapidPublicKeyResponse)
 def get_vapid_public_key(_user: str = Depends(get_current_user)):
     if not settings.VAPID_PUBLIC_KEY:
         raise HTTPException(status_code=503, detail="Push notifications not configured")
     return {"publicKey": settings.VAPID_PUBLIC_KEY}
 
 
-@router.post("/subscribe", status_code=201)
+@router.post("/subscribe", status_code=201, response_model=SubscriptionStatusResponse)
 def subscribe(
     data: PushSubscriptionCreate,
     session: Session = Depends(get_session),
@@ -48,7 +57,7 @@ def subscribe(
     return {"status": "subscribed"}
 
 
-@router.delete("/unsubscribe")
+@router.delete("/unsubscribe", response_model=SubscriptionStatusResponse)
 def unsubscribe(
     data: PushSubscriptionCreate,
     session: Session = Depends(get_session),