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/performance/test_api_performance.py

@@ -9,8 +9,8 @@ import time
 
 import pytest
 
-from models.database.marketplace_product import MarketplaceProduct
-from models.database.marketplace_product_translation import (
+from app.modules.marketplace.models import (
+    MarketplaceProduct,
     MarketplaceProductTranslation,
 )