feat: Firebase Cloud Messaging push notifications

Backend:
- New user_fcm_tokens table (migration 010) for storing device tokens
- FCMService sends push via Firebase HTTP legacy API (server key from .env)
- POST /api/fcm/register endpoint for Android to register its FCM token
- AdminController::sendNotification() triggers FCM push after DB insert
- FCM config added to config.php

Android:
- Firebase Cloud Messaging service (ServerManagerFirebaseService)
  receives push notifications and shows them via NotificationHelper
- FCM token registered with backend on new token and on login
- google-services.json template (must be replaced with real Firebase config)
- firebase-setup.sh script with configuration instructions
- FCM dependency + google-services plugin added to Gradle
- Bump to v1.8.0 (versionCode 12)

src/Controllers/AdminController.php

@@ -11,6 +11,7 @@ use ServerManager\Core\Validator;
 use ServerManager\Models\Notification;
 use ServerManager\Models\User;
 use ServerManager\Services\AuditService;
+use ServerManager\Services\FCMService;
 
 class AdminController
 {
@@ -280,13 +281,20 @@ class AdminController
         }
 
         $notificationModel = new Notification();
-        $notificationModel->create([
+        $noteId = $notificationModel->create([
             'user_id' => $userId,
             'title' => $title,
             'message' => $message,
             'type' => $type,
         ]);
 
+        $fcm = new FCMService();
+        if ($userId) {
+            $fcm->sendToUser($userId, $title, $message, $type, $noteId);
+        } else {
+            $fcm->sendToAll($title, $message, $type, $noteId);
+        }
+
         $this->auditService->log('notification_sent', 'notification', null, ['title' => $title, 'target' => $userId ? "user #{$userId}" : 'all users']);
 
         $this->view->json(['success' => true, 'message' => 'Notification sent successfully.']);