add a common interface for services

atm services are simple POROs. it'd be nice they all return the same object (monad), raise the same error to allow unified way to deal with them from the controller.

https://github.com/Benefactorum/app/pull/212#discussion_r1979466805

Start by reviewing the service implementations and the controller handling discussed in issue #211, then read the linked pull-request discussion for existing context. Define the scope of a common service result and error contract before making changes; done means the services and controller can use that contract consistently.

Written by the indexing model from the issue text.