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)

routes/web.php

@@ -124,6 +124,7 @@ $router->get('/api/profile', [ApiController::class, 'profile']);
 $router->put('/api/profile', [ApiController::class, 'updateProfile']);
 $router->get('/api/refresh-metrics', [ApiController::class, 'refreshAllMetrics']);
 $router->post('/api/metrics/push', [ApiController::class, 'pushMetrics']);
+$router->post('/api/fcm/register', [ApiController::class, 'registerFcmToken']);
 
 $router->post('/servers/:id/agent/install', [ServerController::class, 'agentInstall'], [AuthMiddleware::class, CSRFMiddleware::class]);
 $router->post('/servers/:id/agent/uninstall', [ServerController::class, 'agentUninstall'], [AuthMiddleware::class, CSRFMiddleware::class]);