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>

tests/unit/utils/test_csv_processor.py

@@ -9,7 +9,7 @@ import requests
 import requests.exceptions
 
 from app.utils.csv_processor import CSVProcessor
-from models.database.marketplace_product import MarketplaceProduct
+from app.modules.marketplace.models import MarketplaceProduct
 
 
 @pytest.mark.unit