folder to m3u

**is your feature request related to a problem? Please describe.**
Sometimes I want to queue up a whole album on a remote audio player that uses yt-dlp and mpv (https://git.pvv.ntnu.no/Grzegorz)

I have to either make an m3u and submit the link to that, or copy the file path of every file one by one.

**Describe the idea / solution you'd like**
I would like to be able to do `?m3u` similar to how you can do `?rss`
to get the files in a folder, maybe with an option for recursing into subfolders as an m3u

查看代码库中 ?rss 功能的实现方式，它可能位于请求处理程序或路由模块中。确定生成 RSS feed 的函数，并调整它以生成 M3U 播放列表。检查是否已有用于列出文件的逻辑，以枚举文件夹中的文件，并添加递归支持。通过 Web 界面提供一个文件夹，并在 URL 后追加 ?m3u 进行测试。

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