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/services/product_service.py

@@ -18,8 +18,8 @@ from app.exceptions import (
     ProductNotFoundException,
     ValidationException,
 )
-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
 from models.schema.product import ProductCreate, ProductUpdate
 
 logger = logging.getLogger(__name__)
@@ -272,7 +272,7 @@ class ProductService:
         from sqlalchemy import or_
         from sqlalchemy.orm import joinedload
 
-        from models.database.product_translation import ProductTranslation
+        from app.modules.catalog.models import ProductTranslation
 
         try:
             # Prepare search pattern for LIKE queries