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/features.py

@@ -24,7 +24,7 @@ from app.api.deps import get_current_vendor_api
 from app.core.database import get_db
 from app.exceptions import FeatureNotFoundError
 from app.services.feature_service import feature_service
-from models.database.user import User
+from models.schema.auth import UserContext
 
 router = APIRouter(prefix="/features")
 logger = logging.getLogger(__name__)
@@ -114,7 +114,7 @@ class FeatureCheckResponse(BaseModel):
 
 @router.get("/available", response_model=FeatureCodeListResponse)
 def get_available_features(
-    current_user: User = Depends(get_current_vendor_api),
+    current_user: UserContext = Depends(get_current_vendor_api),
     db: Session = Depends(get_db),
 ):
     """
@@ -148,7 +148,7 @@ def get_available_features(
 def get_features(
     category: str | None = Query(None, description="Filter by category"),
     include_unavailable: bool = Query(True, description="Include features not available to vendor"),
-    current_user: User = Depends(get_current_vendor_api),
+    current_user: UserContext = Depends(get_current_vendor_api),
     db: Session = Depends(get_db),
 ):
     """
@@ -211,7 +211,7 @@ def get_features(
 
 @router.get("/categories", response_model=CategoryListResponse)
 def get_feature_categories(
-    current_user: User = Depends(get_current_vendor_api),
+    current_user: UserContext = Depends(get_current_vendor_api),
     db: Session = Depends(get_db),
 ):
     """
@@ -226,7 +226,7 @@ def get_feature_categories(
 
 @router.get("/grouped", response_model=FeatureGroupedResponse)
 def get_features_grouped(
-    current_user: User = Depends(get_current_vendor_api),
+    current_user: UserContext = Depends(get_current_vendor_api),
     db: Session = Depends(get_db),
 ):
     """
@@ -273,7 +273,7 @@ def get_features_grouped(
 @router.get("/{feature_code}", response_model=FeatureDetailResponse)
 def get_feature_detail(
     feature_code: str,
-    current_user: User = Depends(get_current_vendor_api),
+    current_user: UserContext = Depends(get_current_vendor_api),
     db: Session = Depends(get_db),
 ):
     """
@@ -328,7 +328,7 @@ def get_feature_detail(
 @router.get("/check/{feature_code}", response_model=FeatureCheckResponse)
 def check_feature(
     feature_code: str,
-    current_user: User = Depends(get_current_vendor_api),
+    current_user: UserContext = Depends(get_current_vendor_api),
     db: Session = Depends(get_db),
 ):
     """