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/auth.py

@@ -1,4 +1,5 @@
 from fastapi import APIRouter, Depends, Request
+from pydantic import BaseModel
 from fastapi.security import OAuth2PasswordRequestForm
 
 from app.auth import (
@@ -11,7 +12,16 @@ from app.auth import (
 router = APIRouter(prefix="/auth", tags=["auth"])
 
 
-@router.post("/login")
+class TokenResponse(BaseModel):
+    access_token: str
+    token_type: str
+
+
+class MeResponse(BaseModel):
+    username: str
+
+
+@router.post("/login", response_model=TokenResponse)
 def login(request: Request, form_data: OAuth2PasswordRequestForm = Depends()):
     check_login_rate_limit(request)
     verify_admin_credentials(form_data.username, form_data.password)
@@ -19,6 +29,6 @@ def login(request: Request, form_data: OAuth2PasswordRequestForm = Depends()):
     return {"access_token": token, "token_type": "bearer"}
 
 
-@router.get("/me")
+@router.get("/me", response_model=MeResponse)
 def me(username: str = Depends(get_current_user_cookie_or_bearer)):
     return {"username": username}