Health checks

On by default, on their own port. Config file only.

Health:
  Enabled: true
  Port: 8081

Endpoint	Returns
`GET /healthz`	`200 OK` while the process runs
`GET /readyz`	`200 READY` when MQTT is connected and the PDU was polled recently; else `503 NOT READY: <reason>`
`GET /health/integrations`	JSON status of each integration. Does not affect readiness

The Helm chart uses /healthz and /readyz as liveness and readiness probes (healthProbes.enabled, default on).