Fix CORS Configuration

The cors package is imported in index.js, but it is never used as middleware. This will cause requests from the frontend to fail. Expected Behavior: The application should use app.use(cors()) to allow cross-origin requests.

Start in `index.js`, where `cors` is imported. Locate the app initialization and middleware registration point, and add `app.use(cors())` so CORS is actually enabled before route handling. Run the app (using its normal start command) and verify a cross-origin request path in the API now returns CORS headers instead of failing. Mark done when frontend-origin requests succeed as expected from this issue’s description.

索引モデルが issue の本文から書いたものです。