Fix early reminder notifications and lost recovery notifications

- AlertState.update() now resets last_notification when the alert level
  changes, so a WARNING→CRITICAL escalation restarts the reminder interval
  rather than inheriting a nearly-expired timer.
- _dispatch_to_channel() bypasses min_level for RECOVER, so recovery
  notifications are delivered even after a server restart when
  _alerted_channels is empty and the fallback dispatch path is used.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

hbd/server/threshold.py

@@ -105,6 +105,7 @@ class AlertState:
             self.level = level
             self.since = now
             self.notification_count = 0
+            self.last_notification = None  # restart reminder interval on level change
             # Reset acknowledgment on state change
             if level != AlertLevel.OK:
                 # Only reset if changing to a different alert level