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>

scripts/debug_historical_import.py

@@ -6,7 +6,7 @@ sys.path.insert(0, ".")
 
 from app.core.database import SessionLocal
 from app.services.letzshop.credentials_service import LetzshopCredentialsService
-from models.database.letzshop import LetzshopHistoricalImportJob
+from app.modules.marketplace.models import LetzshopHistoricalImportJob
 
 
 def get_valid_shipment_states(vendor_id: int = 1):