fix(api): add nullable context to #96/#98 DTOs; regen OpenAPI + SPA types

ChannelResponseModel and BulkChannelRequests DTOs lacked file-scoped
#nullable enable, so the OpenAPI generator emitted every string
property as a ["null","string"] union (project Nullable default is
disable), polluting SPA typegen with needless `| null` unions.
ChannelStateResponseModel already complied with this convention
(precedent: PlayoutResponseModel). All properties in both files are
always populated (never null in practice), so no property needed to
become explicitly optional/nullable - just adding #nullable enable
was sufficient to clean up the generated schema.

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

ErsatzTV/Controllers/Api/Requests/BulkChannelRequests.cs

@@ -1,3 +1,4 @@
+#nullable enable
 using ErsatzTV.Application.Channels;
 
 namespace ErsatzTV.Controllers.Api.Requests;