Add HealthService with Liveness (no-op) and Readiness (DB+Redis ping
with per-component timeout) checks. Expose three endpoints:
- /healthz : new liveness endpoint, zero-dependency, always 200
- /ready : new readiness endpoint, returns 503 with details on dep
failure; suitable for K8s readinessProbe and load balancers
- /health : preserved for backward compatibility, equivalent to
/healthz
Switch primary docker-compose healthcheck to /ready so the container
is only marked healthy once DB+Redis are reachable. Standalone/dev/
local compose files keep /health to avoid disrupting existing setups.
Tests: unit tests cover liveness, readiness with both deps healthy,
each dep failing independently, and per-component timeout enforcement.
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