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)

android/app/google-services.json

@@ -0,0 +1,35 @@
+{
+  "project_info": {
+    "project_number": "000000000000",
+    "project_id": "servermanager-placeholder",
+    "storage_bucket": "servermanager-placeholder.appspot.com"
+  },
+  "client": [
+    {
+      "client_info": {
+        "mobilesdk_app_id": "1:000000000000:android:0000000000000000",
+        "android_client_info": {
+          "package_name": "com.devlab.app"
+        }
+      },
+      "oauth_client": [],
+      "api_key": [
+        {
+          "current_key": "PLACEHOLDER_API_KEY_REPLACE_IN_FIREBASE_CONSOLE"
+        }
+      ],
+      "services": {
+        "analytics_service": {
+          "status": 0
+        },
+        "appinvite_service": {
+          "status": 0
+        },
+        "google_cast_service": {
+          "status": 0
+        }
+      }
+    }
+  ],
+  "configuration_version": "1"
+}