The key was committed to three backends behind `|| DEFAULT_KEY` fallbacks, which
also meant a misconfigured deploy silently billed the bundled key instead of
failing. It is now read from the environment only: /api/ask answers 503 and
/api/status reports hasKey:false when it is absent.
hasKey was unreachable-false as well, via `|| true` in the Pages function and
`|| DEFAULT_KEY` in the worker, so it could never signal a missing key.
All responses now go through a shared json() helper, so error paths carry the
same CORS headers and content type as success paths, and wrangler.json declares
the ASSETS binding the worker's static fallback already referenced.
