Add media proxy feature for resource downloading

- Introduced a new API endpoint `GET /api/v1/video/proxy` to facilitate media resource downloads, allowing users to bypass domain restrictions imposed by WeChat.
- Updated configuration to include proxy settings such as `SHORT_VIDEO_PROXY_ENABLED`, `SHORT_VIDEO_PROXY_ALLOWED_DOMAINS`, `SHORT_VIDEO_PROXY_MAX_SIZE_MB`, and `SHORT_VIDEO_PROXY_TIMEOUT_SECONDS`.
- Enhanced the `ShortVideoConfig` struct to accommodate new proxy-related fields.
- Improved error handling for proxy requests, including checks for allowed domains and file size limits.
- Updated documentation to reflect the new proxy functionality and its configuration options, ensuring clarity for users and developers.

.env.example

@@ -23,6 +23,15 @@ JWT_SECRET=your-secret-key-change-in-production
 SHORT_VIDEO_API_KEY=replace-with-real-key
 SHORT_VIDEO_FREE_QUOTA=20
 SHORT_VIDEO_TIMEOUT_SECONDS=5
+# 媒体代理配置（用于中转下载，绕过微信域名限制）
+SHORT_VIDEO_PROXY_ENABLED=true
+# 允许代理的域名白名单（留空表示允许所有，不推荐）
+# 多个域名用逗号分隔，例如：cdn.example.com,video.example.com
+SHORT_VIDEO_PROXY_ALLOWED_DOMAINS=
+# 代理文件最大大小（MB）
+SHORT_VIDEO_PROXY_MAX_SIZE_MB=100
+# 代理请求超时（秒）
+SHORT_VIDEO_PROXY_TIMEOUT_SECONDS=60
 
 # AI 配置（OpenAI-compatible）
 # 例：OpenAI = https://api.openai.com/v1