Warn if out of disk when saving backups

On self-hosted servers: Have the backup script trap errors (in Bash), and use cURL to send out-of-disk webhook messages to configurable chat channels, e.g. Slack, Discord.

Also check for out-of-disk problems on any off-site backup server.

Maybe send disk-low warnings too? But that starts looking like general sysadmin things, hmm.

(I set this to size 5 = about a week. Making the actual change is quick, maybe just a few hours, but verifying that it works is going to take longer. E.g. signing up for some chat app. Also, reading about their webhook handlers etc.
This does not include automated tests (of running out of disk).)