refactor: use Relay::notifications() for event-driven disconnect detection

Replace the 1-second polling loop with nostr-sdk's relay-level notification
system that provides immediate disconnect detection via RelayNotification::RelayStatus.

Key changes:
- Use relay.notifications() instead of client.notifications()
- Handle RelayNotification::RelayStatus { Disconnected | Terminated } to detect
  connection loss immediately without polling
- Remove tokio::select! with interval timer - now uses simple match loop
- Handle additional notification types (Authenticated, AuthenticationFailed)

Why this is better:
- Event-driven vs polling: no wasted CPU cycles checking every second
- Immediate detection: disconnect triggers notification instantly
- Uses nostr-sdk's built-in mechanism that was previously inaccessible at pool level
  (RelayStatus notifications are filtered out in RelayPoolNotification)

Technical note: RelayNotification::RelayStatus is only available via
Relay::notifications(), not Client::notifications(), because the pool-level
broadcast filters out status change events.

Future refactoring opportunity: Consider restructuring RelayConnection to hold
a Relay directly instead of wrapping a Client, since we only manage one relay
per connection anyway.

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