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=
 

backend/app/util/Db.php

@@ -26,7 +26,7 @@ class Db
             'driver' => 'mysql',
             'host' => getenv('DB_HOST') ?: '127.0.0.1',
             'port' => getenv('DB_PORT') ?: '3306',
-            'database' => getenv('DB_DATABASE') ?: 'x_server_mobile',
+            'database' => getenv('DB_DATABASE') ?: 'xserver',
             'username' => getenv('DB_USERNAME') ?: 'root',
             'password' => getenv('DB_PASSWORD') ?: '',
             'charset' => 'utf8mb4',

docker-compose.yml

@@ -12,7 +12,7 @@ services:
     environment:
       - DB_HOST=mysql
       - DB_PORT=3306
-      - DB_DATABASE=x_server_mobile
+      - DB_DATABASE=xserver
       - DB_USERNAME=root
       - DB_PASSWORD=123456
       - REDIS_HOST=redis