Refactor `sync` management command to make it clearer and more maintainable

[jamalex]

The sync command has a lot of branching logic and a variety of scenarios, and is only getting more complex with the introduction of individual user syncing and at some point further KDP integrations (e.g. for loading a facility from KDP by token). The full set of possible syncing scenarios would explode this further (though not all need to be captured directly within the sync command itself).

Refactoring and cleaning up the flow and logic, and ensuring all paths are covered by tests, will be an important item of tech debt to support sustainability of this part of the code.