refactor: update services, alembic, scripts to use module imports

Update remaining application code to use canonical module locations:
- app/services/letzshop/order_service.py
- app/services/platform_health_service.py
- alembic/env.py (inventory and order models)
- scripts/investigate_order.py, verify_setup.py, seed_demo.py

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

scripts/verify_setup.py

@@ -143,7 +143,7 @@ def verify_model_structure():
         )
 
         # Import specific models
-        from models.database.inventory import Inventory
+        from app.modules.inventory.models import Inventory
         from models.database.marketplace_import_job import MarketplaceImportJob
         from models.database.marketplace_product import MarketplaceProduct
         from models.database.product import Product