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_vendor_service.py

@@ -358,7 +358,7 @@ class TestVendorService:
 
     def test_add_product_to_vendor_success(self, db, test_vendor, unique_product):
         """Test successfully adding product to vendor."""
-        from models.database.marketplace_product import MarketplaceProduct
+        from app.modules.marketplace.models import MarketplaceProduct
 
         # Re-query objects to avoid session issues
         vendor = db.query(Vendor).filter(Vendor.id == test_vendor.id).first()
@@ -398,7 +398,7 @@ class TestVendorService:
     def test_add_product_to_vendor_already_exists(self, db, test_vendor, test_product):
         """Test adding product that's already in vendor fails."""
         # Re-query to get fresh instances
-        from models.database.product import Product
+        from app.modules.catalog.models import Product
 
         vendor = db.query(Vendor).filter(Vendor.id == test_vendor.id).first()
         product = db.query(Product).filter(Product.id == test_product.id).first()