feat: config.php 支持 DB_PATH 环境变量，更新部署文档

- db_path 可通过 DB_PATH 环境变量覆盖，本地默认 server/chat.db
- DEPLOY.md 增加 Docker 部署示例和环境变量说明

2026-05-15

server/src/config.php

@@ -1,6 +1,6 @@
 <?php
 return [
-    'db_path' => __DIR__ . '/../chat.db',
+    'db_path' => getenv('DB_PATH') ?: __DIR__ . '/../chat.db',
     'jwt_secret' => getenv('JWT_SECRET') ?: 'chat-terminal-secret-key-change-in-production',
     'password' => getenv('CHAT_PASSWORD') ?: 'admin',
 ];