Adds DecoController (groups + decos CRUD + full-state PUT), playlist and
artist/multi-collection search picker wrappers, a PlaylistController for the
break-content playlist cascade, and PUT /api/playouts/{id}/deco. Fixes the
CreateDecoHandler missing deco-group existence check (FK 500 -> 422), matching
the CreateBlock/CreateTemplate precedent. ReplaceDecoRequest.ToCommand rejects
Merge on the non-mergeable sections (422).
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
