Make backup cleanup failures and other backup rots more visible through notifications

[SansGuidon]

Hey
This relates to a bunch of issues mentioned recently in Support in https://forum.cloudron.io/topic/12535/repeated-backup-failures-with-an-app/10?_=1728054597143

I noticed a few things recently:

• Backups were sometimes failing with a notification.
• My remote backup storage was filling up without knowing.
• Backup cleanup was failing in Cloudron for a long time.
• Some cleanup tasks were queued without notification, it was telling something wrong about the general state of things.
• Backup would suddenly take very long time, going from 30min usually to something like 10 hours and many retries. No notification.

Both were related but I had no idea about those because we get no notification in Cloudron regarding the failures of backup cleanup. It's only when going to backups page then checking the listing of retention that I could see those are failing.

It could prevent disasters if we were notified about those issues which are sadly likely hidden under the carpet for now among a few users.

What do you think ?