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/services/test_inventory_service.py

@@ -363,11 +363,9 @@ class TestInventoryService:
     def test_get_product_inventory_no_inventory(self, db, test_product, test_vendor):
         """Test getting inventory for product with no inventory entries."""
         # Create a new product without inventory
-        from models.database.marketplace_product import MarketplaceProduct
-        from models.database.marketplace_product_translation import (
-            MarketplaceProductTranslation,
-        )
-        from models.database.product import Product
+        from app.modules.marketplace.models import MarketplaceProduct
+        from app.modules.marketplace.models import MarketplaceProductTranslation
+        from app.modules.catalog.models import Product
 
         unique_id = str(uuid.uuid4())[:8]
         mp = MarketplaceProduct(