feat(api): host-agent protocol v2 consumer — heartbeat persistence, auto-register, staleness sweep

Nothing persisted agent heartbeats before: companion_last_seen was
written once at setup and connection_status stayed 'connected' forever.
HostAgentConsumerService now consumes corrosion.*.host.heartbeat
(updates last_seen + status, auto-creates the bare_metal connection row
on first contact), host.going_offline (graceful offline), and sweeps
connections offline after 180s of heartbeat silence. License-existence
tenant validation with caching per NATS-consumer doctrine. WS bridge
forwards host_heartbeat/host_going_offline to the panel.

Contract-verified against production NATS with the backend's own nats
lib: v2 subjects, schema 2, real telemetry, offline beacon.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

backend-nest/src/services/index.ts

@@ -1,3 +1,4 @@
 export { NatsService } from './nats.service';
 export { NatsBridgeService } from './nats-bridge.service';
+export { HostAgentConsumerService } from './host-agent-consumer.service';
 export { SteamService } from './steam.service';