Check for missing files after startup

[t-ober]

Inspired by recent events:

It would be beneficial if the program checks for missed files after a downtime of the server.

This would require the program to after startup check the latest downloads (we seem to save metadata of the downloads already) and determine if there are any files that were missed during the downtime and download the ones still available.

We currently keep such kind of a list and perform the check during runtime but this list is stored in-memory which gets lost if the server gets restarted.