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.

internal/routes/remove_watermark_routes.go

@@ -12,4 +12,6 @@ func registerRemoveWatermarkRoutes(api *gin.RouterGroup, protected *gin.RouterGr
 	protected.POST("/video/remove_watermark/unlock", videoHandler.UnlockQuota)
 	// 下载失败上报：供其他服务调用，无需鉴权
 	api.POST("/video/remove_watermark/report_failure", videoHandler.ReportDownloadFailure)
+	// 媒体代理：用于中转视频/图片等资源，绕过微信域名限制，无需鉴权
+	api.GET("/video/proxy", videoHandler.ProxyMedia)
 }