Implement centralized error handling middleware

Create a centralized Express error handling middleware that converts thrown errors into consistent JSON responses with status codes and error codes. Ensure unexpected errors are logged but do not leak internal details to clients.

Look for existing middleware in the Express app setup, likely in app.js or server.js. Create a new middleware function that catches errors, logs them internally, and sends a structured JSON response with status and error code. Check for any existing error handling utilities or logging configuration. Test by throwing different errors in routes to verify the middleware catches and formats them correctly.

由索引模型根據 Issue 內容生成。