refactor(api): introduce UserContext schema for API dependency injection

Replace direct User database model imports in API endpoints with UserContext
schema, following the architecture principle that API routes should not import
database models directly.

Changes:
- Create UserContext schema in models/schema/auth.py with from_user() factory
- Update app/api/deps.py to return UserContext from all auth dependencies
- Add _get_user_model() helper for functions needing User model access
- Update 58 API endpoint files to use UserContext instead of User
- Add noqa comments for 4 legitimate edge cases (enums, internal helpers)

Architecture validation: 0 errors (down from 61), 11 warnings remain

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/api/v1/vendor/auth.py

@@ -24,7 +24,7 @@ from app.core.environment import should_use_secure_cookies
 from app.exceptions import InvalidCredentialsException
 from app.services.auth_service import auth_service
 from middleware.vendor_context import get_current_vendor
-from models.database.user import User
+from models.schema.auth import UserContext
 from models.schema.auth import LogoutResponse, UserLogin, VendorUserResponse
 
 router = APIRouter(prefix="/auth")
@@ -179,7 +179,7 @@ def vendor_logout(response: Response):
 
 @router.get("/me", response_model=VendorUserResponse)
 def get_current_vendor_user(
-    user: User = Depends(get_current_vendor_api), db: Session = Depends(get_db)
+    user: UserContext = Depends(get_current_vendor_api), db: Session = Depends(get_db)
 ):
     """
     Get current authenticated vendor user.