docs: 📝 update environment configuration and data directory documentation

- Revised `.env.example` to clarify data storage configuration for local development and Docker usage.
- Enhanced `DATA_DIRECTORY.md` with detailed instructions on data persistence, backup, and recovery using Docker Named Volumes.
- Updated `docker-compose.yml` to utilize Named Volume for data storage, ensuring data persistence across container lifecycles.
- Adjusted README to reflect changes in data directory management and deployment instructions.

.env.example

@@ -28,8 +28,9 @@ WEBUI_HOST=0.0.0.0              # WebUI 可对外访问（有登录认证保护
 WEBUI_PORT=7778
 WEBUI_PASSWORD=goproxy          # ⚠️ 生产环境请修改为强密码
 
-# 数据存储
-DATA_DIR=./data
+# 数据存储配置（仅 docker run 本地开发需要）
+# docker-compose.yml 使用 Named Volume，无需此配置
+# DATA_DIR=./data
 
 # 时区配置
 TZ=Asia/Shanghai