[Feature]: mp3 backend storage #783

What problem does this solve?

Currently, stems are extracted to wav files.
With hard drive price not going down in the near future, i'd like to have a global option to store extracted stems in mp3. I don't think that the quality loss of a compressed format is a big deal given the stem extraction artifacts.

Proposed solution

In the settings page, add an option to set the output storage format.
or
an env var (for docker purpose)

Alternatives considered

No response