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/modules/inventory/services/inventory_service.py

@@ -30,7 +30,7 @@ from app.modules.inventory.schemas.inventory import (
     InventoryUpdate,
     ProductInventorySummary,
 )
-from models.database.product import Product
+from app.modules.catalog.models import Product
 from models.database.vendor import Vendor
 
 logger = logging.getLogger(__name__)
@@ -603,8 +603,8 @@ class InventoryService:
             query = query.filter(Inventory.quantity <= low_stock)
 
         if search:
-            from models.database.marketplace_product import MarketplaceProduct
-            from models.database.marketplace_product_translation import (
+            from app.modules.marketplace.models import (
+                MarketplaceProduct,
                 MarketplaceProductTranslation,
             )