Add option to schedule backups

[4jNsY6fCVqZv]

This way, my Planify database could become a regular part of my scheduled system backups without having to manually create a Planify backup each time before.

Planify 4.15.2 (Flatpak)

[redcatbear]

+1 on this feature.

Even the simplest possible implementation of hard-coded once per week would be a big plus for availability.