Auto convert YouTube audio WebA format to opus without re-encoding

[onyx-nxt]

describe the feature you'd like to see

When selecting the "best" (no re-encode) quality in cobalt, downloading audio from YouTube results in a weba file. These just hold an opus stream. It's easy to convert it after in ffmpeg with ffmpeg -i "audio.weba" -c:a copy "audio.opus", but within cobalt with the remux like feature automatically would be nice, since weba isn't super common and also contains wrong metadata for values like bitrate. It's also weirdly 100kb larger for some reason.

additional context

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