Show progress in WebUI

[jtagcat]

Right now it gets dumped to WARN on celery-slow

[CupofDalek]

I too would be interested in this

I had some files hang and had no idea. would be nice to have a progress percentage of some sort visible on web so we can recognize if it has hung or not and if we need to re-queue

[jtagcat]

jfyi I abandoned efforts to run spleeter-web, and made a small thing instead: https://github.com/jtagcat/demucs-web

(though it has no progress either, I'm not bothering to add it as the progress bars given aren't that accurate)