Backend: Add routes-f error normalization

## Summary
Add a Routes-F error normalization layer.

## Requirements
- Standardize error shape `{ code, message, details }`.
- Ensure all routes-f endpoints use it.
- Map common error cases to correct HTTP codes.

## Notes
- Keep error codes enumerated in one place.

## Acceptance Criteria
- Error response shape consistent across routes.
- Tests validate mapping for `400/401/403/404/422/500`.
