Split Backend into an external and internal API

[mschroederi]

The backend should be split into one API handling all user requests (external) and one API handling all cluster-internal requests (put results/ ...)

[boehmchen]

To better follow the principle of separation of concerns, it might be better to create an API for internal requests (requests from other backend applications) and an API for external requests (e.g. frontend requests). Implementing this issue requires rethinking the backend application. (and maybe an entire reimplementation)