Log system events and warn on errors

### Describe the problem

https://github.com/WeblateOrg/weblate/issues/12107 makes it apparent that the database backup has failed in the background, but nobody was reading the logs to notice that.

### Describe the solution you would like

* Weblate should log important system events.
* Initially, it should cover database and settings backup.
* The logs should be browsable in the management interface.
* Old logs should be automatically purged with a configurable period (similar as `ADDON_ACTIVITY_LOG_EXPIRY`).
* There should be a deployment check to verify that important operations are running as scheduled.

### Describe alternatives you have considered

The thing to consider is introducing something more generic to cover site-wide logs for this, backup specific logs (see weblate.wladmin.models.BackupLog), logs for addons (see https://github.com/WeblateOrg/weblate/pull/11080) and logs for automatic translation (see https://github.com/WeblateOrg/weblate/issues/9638). Maybe some of these could actually be based on `weblate.trans.models.change.Change`?

### Screenshots

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### Additional context

_No response_