Currently batch re-encoding or deletion of transcodes is done on the command line with requeueTranscodes.php, but this is awkward and error prone and getting more confusing with maintenance stuff moving into k8s in production.

Once T362123 is resolved and Special:TimedMediaHandler is usable again I want to put a better control interface for background transcode jobs on there:

• add a table of ongoing jobs with (job type, user, start time, job options, current position through input data, cancellation request state)
• display live jobs
• have a form for adding new jobs
• allow canceling ongoing jobs, at least at the precision of when they finish the current file they check the table for the state and can stop instead of enqueueing another job
• controllable fan-out setting for multiprocess