Error handling middleware

Centralize JSON error responses with consistent shape `{error:{message,code}}`.

Look for existing error handling in the Express app setup, likely in app.js or a middleware directory. Identify where errors are currently thrown and how they are formatted. Create a middleware function that catches errors, wraps them in the specified JSON shape, and sets appropriate HTTP status codes. Test by triggering different error conditions and verifying the response format.

索引モデルが issue の本文から書いたものです。