[RESEARCH] Find a canonical way to split the logic between "service" and "controller"

[AlexPykavy]

Questions to clarify:

• Should a service return nil or an error if there is no entity in the store. Or "service" may return nil but the upstream controller should return 404?
• Should a controller return 404 if we are trying to delete a non-existent entity?
• Should the list method of a service return a iter.Seq[T] or []T?
• and others