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/investigate_order.py

@@ -5,7 +5,7 @@ import sys
 sys.path.insert(0, ".")
 
 from app.core.database import SessionLocal
-from models.database.order import Order, OrderItem
+from app.modules.orders.models import Order, OrderItem
 
 
 def investigate_order(order_number: str):