* feat: add outbound heartbeat monitoring to external endpoints
Allow Beszel hub to periodically ping an external monitoring service
(e.g. BetterStack, Uptime Kuma, Healthchecks.io) with system status
summaries, enabling monitoring without exposing Beszel to the internet.
Configuration via environment variables:
- BESZEL_HUB_HEARTBEAT_URL: endpoint to ping (required to enable)
- BESZEL_HUB_HEARTBEAT_INTERVAL: seconds between pings (default: 60)
- BESZEL_HUB_HEARTBEAT_METHOD: HTTP method - POST/GET/HEAD (default: POST)
