Add expiry + clean up job for exported files

[fm3]

Exported files are stored on the server (for now a zipfile each) – those should not stay there indefinitely, we should introduce expiry and a clean up job.

The expiry should also be communicated to the user.

Also, intermediate files in .converting should be cleaned up periodically

The question is where to store the expiry timestamps (they need to survive backend restarts). Redis again? Or directly on disk in filenames/additional files?

We could also use modified timestamps from the filesystem.