fix: wire up relay disconnection detection for metrics

- Add periodic health check in RelayConnection::run_event_loop that polls
  nostr-sdk's relay.is_connected() every second to detect dead connections
- When event channel closes without explicit Closed/Shutdown, send
  DisconnectNotification to SyncManager (fixes case where TCP drops silently)
- Enable test_relay_connected_status test which validates the
  ngit_sync_relay_connected metric correctly reflects connection state

The issue was that when a remote relay stops abruptly, nostr-sdk's
notification receiver blocks indefinitely waiting for data. TCP disconnect
detection without keepalive can take minutes. The health check polls
nostr-sdk's internal relay status which detects disconnection promptly.

0 files changed, 0 insertions, 0 deletions