* 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)
- Add /smart route to view SMART data across all systems
- Store SMART devices in new smart_devices collection
- Auto-fetch SMART data when system first comes online
- Add refresh/delete actions per device with realtime updates
- Add navbar and command palette entries
