Kubernetes-style readiness probe. Returns 200 only when the service is ready to serve traffic.
application/json, application/x-protobuf Service is ready to accept traffic.
Readiness probe response returned by GET /health/readiness.
Returns 200 OK only when all downstream dependencies are reachable.
Returns 503 Service Unavailable when any dependency is unhealthy -- the
response body still uses this schema, with ready: false.