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/marketplace.py

@@ -17,7 +17,7 @@ from app.services.marketplace_import_job_service import marketplace_import_job_s
 from app.services.vendor_service import vendor_service
 from app.tasks.background_tasks import process_marketplace_import
 from middleware.decorators import rate_limit
-from models.database.user import User
+from models.schema.auth import UserContext
 from app.modules.marketplace.schemas import (
     MarketplaceImportJobRequest,
     MarketplaceImportJobResponse,
@@ -32,7 +32,7 @@ logger = logging.getLogger(__name__)
 async def import_products_from_marketplace(
     request: MarketplaceImportJobRequest,
     background_tasks: BackgroundTasks,
-    current_user: User = Depends(get_current_vendor_api),
+    current_user: UserContext = Depends(get_current_vendor_api),
     db: Session = Depends(get_db),
 ):
     """Import products from marketplace CSV with background processing (Protected).
@@ -96,7 +96,7 @@ async def import_products_from_marketplace(
 @router.get("/imports/{job_id}", response_model=MarketplaceImportJobResponse)
 def get_marketplace_import_status(
     job_id: int,
-    current_user: User = Depends(get_current_vendor_api),
+    current_user: UserContext = Depends(get_current_vendor_api),
     db: Session = Depends(get_db),
 ):
     """Get status of marketplace import job (Protected)."""
@@ -113,7 +113,7 @@ def get_marketplace_import_jobs(
     marketplace: str | None = Query(None, description="Filter by marketplace"),
     skip: int = Query(0, ge=0),
     limit: int = Query(50, ge=1, le=100),
-    current_user: User = Depends(get_current_vendor_api),
+    current_user: UserContext = Depends(get_current_vendor_api),
     db: Session = Depends(get_db),
 ):
     """Get marketplace import jobs for current vendor (Protected)."""