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/onboarding.py

@@ -20,7 +20,7 @@ from app.api.deps import get_current_vendor_api
 from app.core.database import get_db
 from app.services.onboarding_service import OnboardingService
 from app.tasks.letzshop_tasks import process_historical_import
-from models.database.user import User
+from models.schema.auth import UserContext
 from app.modules.marketplace.schemas import (
     CompanyProfileRequest,
     CompanyProfileResponse,
@@ -49,7 +49,7 @@ logger = logging.getLogger(__name__)
 
 @router.get("/status", response_model=OnboardingStatusResponse)
 def get_onboarding_status(
-    current_user: User = Depends(get_current_vendor_api),
+    current_user: UserContext = Depends(get_current_vendor_api),
     db: Session = Depends(get_db),
 ):
     """
@@ -69,7 +69,7 @@ def get_onboarding_status(
 
 @router.get("/step/company-profile")
 def get_company_profile(
-    current_user: User = Depends(get_current_vendor_api),
+    current_user: UserContext = Depends(get_current_vendor_api),
     db: Session = Depends(get_db),
 ):
     """
@@ -84,7 +84,7 @@ def get_company_profile(
 @router.post("/step/company-profile", response_model=CompanyProfileResponse)
 def save_company_profile(
     request: CompanyProfileRequest,
-    current_user: User = Depends(get_current_vendor_api),
+    current_user: UserContext = Depends(get_current_vendor_api),
     db: Session = Depends(get_db),
 ):
     """
@@ -118,7 +118,7 @@ def save_company_profile(
 @router.post("/step/letzshop-api/test", response_model=LetzshopApiTestResponse)
 def test_letzshop_api(
     request: LetzshopApiTestRequest,
-    current_user: User = Depends(get_current_vendor_api),
+    current_user: UserContext = Depends(get_current_vendor_api),
     db: Session = Depends(get_db),
 ):
     """
@@ -136,7 +136,7 @@ def test_letzshop_api(
 @router.post("/step/letzshop-api", response_model=LetzshopApiConfigResponse)
 def save_letzshop_api(
     request: LetzshopApiConfigRequest,
-    current_user: User = Depends(get_current_vendor_api),
+    current_user: UserContext = Depends(get_current_vendor_api),
     db: Session = Depends(get_db),
 ):
     """
@@ -162,7 +162,7 @@ def save_letzshop_api(
 
 @router.get("/step/product-import")
 def get_product_import_config(
-    current_user: User = Depends(get_current_vendor_api),
+    current_user: UserContext = Depends(get_current_vendor_api),
     db: Session = Depends(get_db),
 ):
     """
@@ -177,7 +177,7 @@ def get_product_import_config(
 @router.post("/step/product-import", response_model=ProductImportConfigResponse)
 def save_product_import_config(
     request: ProductImportConfigRequest,
-    current_user: User = Depends(get_current_vendor_api),
+    current_user: UserContext = Depends(get_current_vendor_api),
     db: Session = Depends(get_db),
 ):
     """
@@ -208,7 +208,7 @@ def save_product_import_config(
 def trigger_order_sync(
     request: OrderSyncTriggerRequest,
     background_tasks: BackgroundTasks,
-    current_user: User = Depends(get_current_vendor_api),
+    current_user: UserContext = Depends(get_current_vendor_api),
     db: Session = Depends(get_db),
 ):
     """
@@ -253,7 +253,7 @@ def trigger_order_sync(
 )
 def get_order_sync_progress(
     job_id: int,
-    current_user: User = Depends(get_current_vendor_api),
+    current_user: UserContext = Depends(get_current_vendor_api),
     db: Session = Depends(get_db),
 ):
     """
@@ -271,7 +271,7 @@ def get_order_sync_progress(
 @router.post("/step/order-sync/complete", response_model=OrderSyncCompleteResponse)
 def complete_order_sync(
     request: OrderSyncCompleteRequest,
-    current_user: User = Depends(get_current_vendor_api),
+    current_user: UserContext = Depends(get_current_vendor_api),
     db: Session = Depends(get_db),
 ):
     """