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/ApiController.php

@@ -13,6 +13,7 @@ use ServerManager\Services\MonitoringService;
 use ServerManager\Services\SSHService;
 use ServerManager\Services\AuditService;
 use ServerManager\Services\PermissionValidator;
+use ServerManager\Services\FCMService;
 use ServerManager\Models\CommandHistory;
 use ServerManager\Core\Validator;
 
@@ -696,6 +697,34 @@ class ApiController
         $this->view->json(['success' => true]);
     }
 
+    public function registerFcmToken(): void
+    {
+        $user = $this->authenticateRequest();
+
+        $input = json_decode(file_get_contents('php://input'), true) ?? $_POST;
+        $token = $input['token'] ?? '';
+
+        if (empty($token)) {
+            $this->view->json(['error' => 'Token is required'], 400);
+        }
+
+        $db = \ServerManager\Core\Database::getInstance();
+        $existing = $db->fetch(
+            'SELECT id FROM user_fcm_tokens WHERE user_id = ? AND token = ?',
+            [(int) $user['id'], $token]
+        );
+
+        if (!$existing) {
+            $db->insert('user_fcm_tokens', [
+                'user_id' => (int) $user['id'],
+                'token' => $token,
+                'created_at' => date('Y-m-d H:i:s'),
+            ]);
+        }
+
+        $this->view->json(['success' => true]);
+    }
+
     public function serverServices(int $id): void
     {
         $user = $this->authenticateRequest();