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/integration/api/v1/vendor/test_dashboard.py

@@ -67,8 +67,8 @@ class TestVendorDashboardAPI:
         """Test that dashboard stats only show data for the authenticated vendor"""
         import uuid
 
-        from models.database.marketplace_product import MarketplaceProduct
-        from models.database.product import Product
+        from app.modules.marketplace.models import MarketplaceProduct
+        from app.modules.catalog.models import Product
 
         # Create products for the test vendor
         for i in range(3):
@@ -195,8 +195,8 @@ class TestVendorDashboardAPI:
         """Test dashboard stats accuracy with actual products"""
         import uuid
 
-        from models.database.marketplace_product import MarketplaceProduct
-        from models.database.product import Product
+        from app.modules.marketplace.models import MarketplaceProduct
+        from app.modules.catalog.models import Product
 
         # Create 5 different marketplace products
         marketplace_products = []