Put Cost Item

Create an item, or change one from a month on (global admin). Body: the item, plus optional from (YYYY-MM, default the current Berlin month, never earlier: 422 from_in_past) — the first month the new figures apply; earlier months keep the old ones. The one way to change a past month is an explicit correction: correction: true + reason (recorded on the event). A credential link must name a fleet credential that carries a cost (422 unknown_credential). 201 on create, 200 on change.