This refactors a part of the database and adds a new "sync" command that reuses a lot of the backup logic to take a set of sources and apply them to a backend.
Unlike the backup process, the sync copies files verbatim without any encryption.
This PR has a large blast radius because it takes the final step and bumps up the Controller to be fully async.
We have historically done a piece-by-piece update, so all operations were already async but the controller interface was kept synchronous.
With this update, the controller is now fully async and all tests are updated.
Most places where the new C# compiler warns about function names not ending in `Async` were also adressed, giving a massive refactor change.
Functionally, no changes are done.
Kenneth Skovhede