- Add offline cache with disk persistence
- Flush cached payloads on successful heartbeat
- Store failed payloads to cache for retry
- Configurable cache path and max size
- Load cache on startup
- Add HF_MONITOR_API_KEY environment variable support
- Add X-API-Key header to HTTP requests
- Remove challenge_uuid dependency
- Use heartbeat-v2 endpoint for API Key auth
- Warn instead of exit when API key is missing