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. This does not include automated tests (of running out of disk).)