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)

database/migrations/010_user_fcm_tokens.sql

@@ -0,0 +1,10 @@
+CREATE TABLE IF NOT EXISTS `user_fcm_tokens` (
+    `id`         INT UNSIGNED NOT NULL AUTO_INCREMENT,
+    `user_id`    INT UNSIGNED NOT NULL,
+    `token`      VARCHAR(255) NOT NULL,
+    `created_at` DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP,
+    `updated_at` DATETIME NOT NULL DEFAULT CURRENT_TIMESTAMP ON UPDATE CURRENT_TIMESTAMP,
+    PRIMARY KEY (`id`),
+    UNIQUE KEY `idx_user_token` (`user_id`, `token`),
+    KEY `idx_user_id` (`user_id`)
+) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_unicode_ci;