- flake.nix: add nodejs_22, firebase-tools, and google-cloud-sdk to the dev
shell so the relay can be tested, built, deployed, and administered locally
- backend settings: point default_relay_url at the deployed relay
- rename the relay liveness route /healthz -> /health: Google Front End
reserves /healthz and returns its own 404 before the request reaches Cloud
Run, so the probe was unreachable (verified live; /v1/push and the FCM path
work end-to-end)
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
