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-mobile-phone-page.php

@@ -55,17 +55,32 @@ class TWP_Mobile_Phone_Page {
             $user_id, $fcm_token
         ));
 
-        if (!$existing) {
+        if ($existing) {
+            // Refresh the expiry on existing session
+            $wpdb->update($table,
+                array('expires_at' => date('Y-m-d H:i:s', time() + 7 * DAY_IN_SECONDS)),
+                array('id' => $existing->id),
+                array('%s'),
+                array('%d')
+            );
+        } else {
             $wpdb->insert($table, array(
-                'user_id'   => $user_id,
-                'fcm_token' => $fcm_token,
-                'device_info' => 'WebView Mobile App',
-                'is_active' => 1,
-                'created_at' => current_time('mysql'),
-                'expires_at' => date('Y-m-d H:i:s', time() + 7 * DAY_IN_SECONDS),
+                'user_id'       => $user_id,
+                'refresh_token' => 'webview-' . wp_generate_password(32, false),
+                'fcm_token'     => $fcm_token,
+                'device_info'   => 'WebView Mobile App',
+                'is_active'     => 1,
+                'created_at'    => current_time('mysql'),
+                'expires_at'    => date('Y-m-d H:i:s', time() + 7 * DAY_IN_SECONDS),
             ));
+
+            if ($wpdb->last_error) {
+                error_log('TWP FCM: Failed to insert token: ' . $wpdb->last_error);
+                wp_send_json_error('Failed to store token');
+            }
         }
 
+        error_log("TWP FCM: Token registered for user $user_id");
         wp_send_json_success('FCM token registered');
     }