feat(api): flat schedule-item DTO + discovery endpoints (#126 #207 #212)

Task A (#126): new non-polymorphic ScheduleItemResponseModel /
ScheduleItemsResponseModel in Core/Api/Scheduling, plus shared
NamedIdResponseModel. ScheduleItemResponseMapper flattens the
One/Flood/Multiple/Duration VM hierarchy. ScheduleController GET/POST/PUT
items now return the flat DTOs.

Task B: GET /api/languages (LanguagesController + LanguageCodeResponseModel);
GET /api/channels/music-video-credits-templates and
GET /api/channels/stream-selectors; FillerKind added to
FillerPresetResponseModel with optional ?fillerKind= filter on
GET /api/filler-presets.

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

ErsatzTV/Controllers/Api/FillerPresetController.cs

@@ -3,6 +3,7 @@ using ErsatzTV.Application.Filler;
 using ErsatzTV.Controllers.Api.Requests;
 using ErsatzTV.Core;
 using ErsatzTV.Core.Api.Filler;
+using ErsatzTV.Core.Domain.Filler;
 using ErsatzTV.Extensions;
 using MediatR;
 using Microsoft.AspNetCore.Http;
@@ -16,10 +17,13 @@ public class FillerPresetController(IMediator mediator) : ControllerBase
     [HttpGet("/api/filler-presets", Name = "GetFillerPresets")]
     [Tags("Filler Presets")]
     [EndpointSummary("Get all filler presets")]
+    [EndpointDescription("Optionally filter to a single filler kind via the fillerKind query parameter.")]
     [EndpointGroupName("general")]
     [ProducesResponseType(typeof(List<FillerPresetResponseModel>), StatusCodes.Status200OK)]
-    public async Task<List<FillerPresetResponseModel>> GetAll(CancellationToken cancellationToken) =>
-        await mediator.Send(new GetAllFillerPresetsForApi(), cancellationToken);
+    public async Task<List<FillerPresetResponseModel>> GetAll(
+        [FromQuery] FillerKind? fillerKind,
+        CancellationToken cancellationToken) =>
+        await mediator.Send(new GetAllFillerPresetsForApi(fillerKind), cancellationToken);
 
     [HttpGet("/api/filler-presets/{id:int}", Name = "GetFillerPresetById")]
     [Tags("Filler Presets")]