conversions should now *actually* stop and terminate when removed, instead of continuing to run in the background until finished.
most notably, magick has been reworked to run a new worker for each conversion to follow ffmpeg and pandoc (& to allow individual cancellations)
also fix uh, a lot of stuff relating to messages not following WorkerMessage type & types in general.
i'm braindead right now but everything still works somehow, vertd is next. this took forever.