Generate downloadable mp4s with embedded multilanguage closed captions

We will need an ffmpeg task to create the mp4
https://video.stackexchange.com/questions/22197/ffmpeg-how-to-add-several-subtitle-streams
We will want to add the audio description/enhanced text description track too.

(We could even add chapter markers from the epub data)

For discussion: How do we not litter the storage with old mp4s?

Some tricky timing corner cases to work out (e.g. captions being updated while mp4 being regenerated)