config: update all database references to xserver

- Update backend/.env: DB_DATABASE=xserver
- Update backend/.env.example: DB_DATABASE=xserver
- Update docker-compose.yml: DB_DATABASE=xserver
- Update backend/app/util/Db.php: default database=xserver

The old database name 'x_server_mobile' has been fully migrated to 'xserver'.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/.env.example

@@ -5,7 +5,7 @@ APP_ENV=local
 # 数据库配置
 DB_HOST=127.0.0.1
 DB_PORT=3306
-DB_DATABASE=x_server_mobile
+DB_DATABASE=xserver
 DB_USERNAME=root
 DB_PASSWORD=