fix: FCM push not sending — add v1 API + better diagnostics

- FCMService now supports both HTTP v1 API (service account) and legacy API (server key)
- v1 API uses OAuth2 JWT auth with Firebase service account; tries this first
- Falls back to legacy HTTP API if only server key is configured
- Removes invalid tokens from DB automatically (NotRegistered, INVALID_ARGUMENT, etc.)
- AdminController now logs push result status and shows in UI response
- .env updated with FCM_SERVER_KEY placeholder and docs
- Migration 010 run in dev

src/Controllers/AdminController.php

@@ -289,14 +289,21 @@ class AdminController
         ]);
 
         $fcm = new FCMService();
+        $pushResult = '';
         if ($userId) {
-            $fcm->sendToUser($userId, $title, $message, $type, $noteId);
+            $sent = $fcm->sendToUser($userId, $title, $message, $type, $noteId);
+            $pushResult = $sent ? ' (push sent)' : ($fcm->isConfigured() ? ' (push failed)' : '');
         } else {
-            $fcm->sendToAll($title, $message, $type, $noteId);
+            $count = $fcm->sendToAll($title, $message, $type, $noteId);
+            $pushResult = $fcm->isConfigured() ? " (push sent to {$count} devices)" : '';
         }
 
-        $this->auditService->log('notification_sent', 'notification', null, ['title' => $title, 'target' => $userId ? "user #{$userId}" : 'all users']);
+        $this->auditService->log('notification_sent', 'notification', null, [
+            'title' => $title,
+            'target' => $userId ? "user #{$userId}" : 'all users',
+            'push' => $fcm->isConfigured() ? 'sent' : 'not_configured',
+        ]);
 
-        $this->view->json(['success' => true, 'message' => 'Notification sent successfully.']);
+        $this->view->json(['success' => true, 'message' => 'Notification sent successfully.' . $pushResult]);
     }
 }