Config: Disable all backups with `PHOTOPRISM_DISABLE_BACKUPS`

[lastzero]

As an admin, I want to be able to disable all bakups with a single config option so I don't need to search for all related options and disable them individually.

Even though the PHOTOPRISM_DISABLE_BACKUPS flag had been deprecated in favor of more fine-grained config options, there may be cases where you want to disable all backups, including database and albums backups as well as YAML sidecar files.

This may be the case, for example, if the sidecar folder is not writable due to a configuration error or on test/demo instances.

Related Issues:

• Backups: Schedule automatic index database and album backups #4243
• Config: Add backup options to the Advanced Settings UI #4269