refactor: migrate models to canonical module locations

- Move Product/ProductTranslation to app/modules/catalog/models/
- Move VendorOnboarding to app/modules/marketplace/models/
- Delete legacy re-export files for marketplace models:
  - letzshop.py, marketplace.py, marketplace_product.py
  - marketplace_product_translation.py, marketplace_import_job.py
- Delete legacy product.py, product_translation.py, onboarding.py
- Update all imports across services, tasks, tests to use module locations

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

app/tasks/background_tasks.py

@@ -7,7 +7,7 @@ from datetime import UTC, datetime
 from app.core.database import SessionLocal
 from app.services.admin_notification_service import admin_notification_service
 from app.utils.csv_processor import CSVProcessor
-from models.database.marketplace_import_job import MarketplaceImportJob
+from app.modules.marketplace.models import MarketplaceImportJob
 from models.database.vendor import Vendor
 
 logger = logging.getLogger(__name__)

app/tasks/letzshop_tasks.py

@@ -11,7 +11,7 @@ from app.services.letzshop import LetzshopClientError
 from app.services.letzshop.credentials_service import LetzshopCredentialsService
 from app.services.letzshop.order_service import LetzshopOrderService
 from app.services.letzshop.vendor_sync_service import LetzshopVendorSyncService
-from models.database.letzshop import LetzshopHistoricalImportJob
+from app.modules.marketplace.models import LetzshopHistoricalImportJob
 
 logger = logging.getLogger(__name__)