Fix FCM token registration and add queue reminder alerts

- Fix silent insert failure in FCM token registration (missing NOT NULL
  refresh_token column) so WebView app tokens are actually stored
- Add 1-minute queue reminder cron that re-sends FCM alerts for calls
  still waiting, with transient-based throttle to prevent duplicates
- Send FCM cancel on queue dequeue (answered/hangup/timeout), not just
  on final call status webhook
- Clean up new cron hook on plugin deactivation

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

includes/class-twp-deactivator.php

@@ -11,6 +11,7 @@ class TWP_Deactivator {
         // Clear scheduled events
         wp_clear_scheduled_hook('twp_check_schedules');
         wp_clear_scheduled_hook('twp_process_queue');
+        wp_clear_scheduled_hook('twp_queue_reminders');
         wp_clear_scheduled_hook('twp_auto_revert_agents');
         
         // Flush rewrite rules