feat(api): expose PlayoutItem row id on the playout-items list DTO (#210)

The SPA needs the item's row id to call
GET /api/playouts/items/{id}/scheduling-context. Plumbed through
PlayoutItemViewModel -> PlayoutItemResponseModel as a nullable Id
(null for synthesized UNSCHEDULED gap rows, which are PlayoutGaps,
not PlayoutItems). Additive for existing consumers (Playouts.razor
reads the VM by property). Regenerated v1.json + v1.d.ts.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

ErsatzTV/Controllers/Api/PlayoutController.cs

@@ -743,6 +743,7 @@ public class PlayoutController(IMediator mediator) : ControllerBase
 
     private static PlayoutItemResponseModel ToItemResponse(PlayoutItemViewModel vm) =>
         new(
+            vm.Id,
             vm.Title,
             vm.Start,
             vm.Finish,