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/analytics.py

@@ -20,7 +20,7 @@ from app.core.database import get_db
 from app.core.feature_gate import RequireFeature
 from app.services.stats_service import stats_service
 from app.modules.billing.models import FeatureCode
-from models.database.user import User
+from models.schema.auth import UserContext
 from app.modules.analytics.schemas import (
     VendorAnalyticsCatalog,
     VendorAnalyticsImports,
@@ -35,7 +35,7 @@ logger = logging.getLogger(__name__)
 @router.get("", response_model=VendorAnalyticsResponse)
 def get_vendor_analytics(
     period: str = Query("30d", description="Time period: 7d, 30d, 90d, 1y"),
-    current_user: User = Depends(get_current_vendor_api),
+    current_user: UserContext = Depends(get_current_vendor_api),
     db: Session = Depends(get_db),
     _: None = Depends(RequireFeature(FeatureCode.BASIC_REPORTS, FeatureCode.ANALYTICS_DASHBOARD)),
 ):