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-core.php

@@ -265,6 +265,9 @@ class TWP_Core {
         
         $queue = new TWP_Call_Queue();
         $this->loader->add_action('twp_process_queue', $queue, 'process_waiting_calls');
+
+        // Queue reminder alerts (re-send FCM every minute for waiting calls)
+        add_action('twp_queue_reminders', array('TWP_Call_Queue', 'send_queue_reminders'));
         
         // Callback processing
         $this->loader->add_action('twp_process_callbacks', 'TWP_Callback_Manager', 'process_callbacks');
@@ -281,6 +284,10 @@ class TWP_Core {
             wp_schedule_event(time(), 'twp_every_30_seconds', 'twp_process_queue');
         }
         
+        if (!wp_next_scheduled('twp_queue_reminders')) {
+            wp_schedule_event(time(), 'twp_every_minute', 'twp_queue_reminders');
+        }
+
         if (!wp_next_scheduled('twp_process_callbacks')) {
             wp_schedule_event(time(), 'twp_every_minute', 'twp_process_callbacks');
         }