Add health check endpoint for Casa

[yurtzy]

Casa exposes your home base to the Clawd network via reverse proxy. It would be useful to have a simple HTTP health check endpoint (/health or /status) that returns JSON indicating the proxy status, connected nodes, and last heartbeat.

This would allow external monitoring (UptimeRobot, cron health checks) to verify the Casa tunnel is up and responsive.

Proposed response:

{
  "status": "healthy",
  "tunnel_active": true,
  "nodes_connected": 2,
  "last_heartbeat": "2026-03-24T08:55:00Z"
}

Endpoint: GET /health (no auth required, or optionally token-based)