Improve "backup success" notification spam

[pcrockett]

Is your feature request related to a problem? Please describe.

My Nextcloud administrator account gets spammed with "backup success" notifications every day that I don't need. Daily notifications about successful backups are not only unnecessary to me; they also contribute to notification / alert fatigue, making it more difficult to separate signal from noise in my daily life.

To be clear, I absolutely DO want to be notified about failed or skipped backups.

Describe the solution you'd like

For other systems I administer, when possible, I send "job success" notifications to services like Cronitor, Dead Man's Snitch, or Healthchecks.io via a simple curl command. These services then only send me a notification when they notice that the successful jobs STOP happening.

It would be ideal if Nextcloud could have a similar behavior: Only notify when backups stop being successful.

Describe alternatives you've considered

• A solution that would be good enough (for me, at least) would be simply to delete this block of code.
• I do remember a day when I was a young warthog, when I did actually check these kinds of daily notifications. So I understand leaving it in there. In that case, it would be nice if there was a checkbox that would allow us to opt out.
• Add a new feature that allows the administrator to configure a healthcheck URL that gets pinged after every successful backup, which would allow a third party service to monitor the health of backups.

Additional context

Would also involve some updates to the content under "Daily backup and automatic updates."

[szaimen]

Will be possible with #3516

[szaimen]

This is now released with v7.5.0 Beta. Testing and feedback is welcome! See https://github.com/nextcloud/all-in-one#how-to-switch-the-channel