refactor: move transaction management from services to API endpoints

- Services now use db.flush() instead of db.commit() for database operations
- API endpoints handle transaction commit after service calls
- Remove db.rollback() from services (let exception handlers manage this)
- Ensures consistent transaction boundaries at API layer

This pattern gives API endpoints full control over when to commit,
allowing for better error handling and potential multi-operation transactions.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

app/api/v1/shop/orders.py

@@ -68,6 +68,7 @@ def place_order(
     order = order_service.create_order(
         db=db, vendor_id=vendor.id, order_data=order_data
     )
+    db.commit()
 
     logger.info(
         f"Order {order.order_number} placed for vendor {vendor.subdomain}, "