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)

config/config.php

@@ -60,6 +60,10 @@ return [
         'rate_limit_window' => (int) ($_ENV['API_RATE_LIMIT_WINDOW'] ?? 60),
     ],
 
+    'fcm' => [
+        'server_key' => $_ENV['FCM_SERVER_KEY'] ?? '',
+    ],
+
     'log' => [
         'path' => $_ENV['LOG_PATH'] ?? __DIR__ . '/../logs/',
         'level' => $_ENV['LOG_LEVEL'] ?? 'warning',