Tri-state backend reachability (M70c).

ioredis auto-reconnects via its default retry strategy, so the broker does not run a reconnect loop of its own; the truth is what ping() reports right now. true when ping() resolves, false when it rejects (the server is down or the socket is mid-reconnect), undefined when the injected client exposes no ping (a minimal fake) — in which case the indicator reports reachable: 'unknown' rather than lying.