API: The AddDummyData() helper function should use the API instead of writing into the DB

[omarkohl]

Otherwise it becomes difficult to enforce the correct DB state e.g. with commit dc609ae Brier score calculation seemed to be done but the dummy data Will "The Fabelmans" win "Best Picture" at the Oscars 2023? forecast had the Brier score set to null despite being resolved.

cleosrv/graph/no_production.go

[omarkohl]

Ideally only once #276 is done because some of the dummy data contains multiple estimates per forecast, which can't be done with the API yet.