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

Look at how the ?rss feature is implemented in the codebase, likely in a request handler or routing module. Identify the function that generates RSS feeds and adapt it to produce M3U playlists. Check for existing file listing logic to enumerate files in a folder, and add recursion support. Test by serving a folder via the web interface and appending ?m3u to the URL.

Written by the indexing model from the issue text.