Shallow liveness probe

The load balancer's liveness probe. Unauthenticated, like the SIWE login routes. Always returns 200; a failed Redis ping is reported as status: "degraded" / healthy: false in the body rather than as a non-2xx status, so the probe never removes an instance that is still serving. The deeper probes (GET /health/deep, GET /executor/dead_letters, GET /internal/canary-health) are gated by internal service tokens and are not reachable with an API key or a session JWT.