backend-error-handlinglisted

## Overview Catch unhandled errors centrally and return consistent JSON error envelopes. ## Steps 1. Register an `app.onError` handler that maps known error types to HTTP status codes. 2. Never leak stack traces in production responses; log them server side instead. 3. Return a stable `{ error: { code, message } }` shape. ## Verify Run the API tests including a route that throws, and confirm the response is a clean JSON envelope with the right status.