System disk usage should include database consumption of each app

[d19dotca]

I'm not sure how feasible this is, but it'd be really helpful to know from the System > Disk Usage list just how much total space is used by an app -- and this should include each app's database usage.

This stems from a concern I have when using Umami which never shows consuming more than 8 Kb of disk space despite the database naturally growing in size, currently it is only ever totalling the disk usage of the /app/data/* directory rather than including the associated database. IMO, the current method doesn't provide the complete picture but only a partial picture of disk usage when detailed at the app level.