feat(api): media detail + info + image-folder endpoints (#141/#161)

Add REST endpoints backing the SPA media detail pages and image browser:

- GET /api/movies/{id}, /api/shows/{id}, /api/seasons/{id}, /api/artists/{id}
  wrapping the existing detail queries; 404 on None.
- GET /api/media-items/{id}/info wrapping GetMediaItemInfo; UnableToLocateMediaItem
  -> 404, other errors -> 422.
- GET /api/images/folders?parentId= and PUT /api/images/folders/{id}/duration
  (validates null-or-positive -> 400; existence guard via new ImageFolderExists
  query -> 404).
- Extend GetLibraryBrowseItems parentId drill-in to Episode (episodes of a season,
  episode-number order) and MusicVideo (an artist's music videos, album/track order),
  alongside the existing TelevisionSeason branch.

Response DTOs live in ErsatzTV.Core/Api/* and never expose Application VMs. Artwork
values are rooted for the SPA via a shared ErsatzTV.Core/Api/ApiArtwork helper
(mirrors the #180/#181 browse-handler logic; handles jellyfin/emby proxy prefixes,
http passthrough, empty). Regenerated OpenAPI v1.json + web v1.d.ts. New controllers
registered in ApiControllerSecurityTests.

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

ErsatzTV/Controllers/Api/ArtistsController.cs

@@ -0,0 +1,39 @@
+using ErsatzTV.Application.Artists;
+using ErsatzTV.Core.Api;
+using ErsatzTV.Core.Api.Artists;
+using ErsatzTV.Core.Domain;
+using ErsatzTV.Extensions;
+using MediatR;
+using Microsoft.AspNetCore.Http;
+using Microsoft.AspNetCore.Mvc;
+
+namespace ErsatzTV.Controllers.Api;
+
+[ApiController]
+public class ArtistsController(IMediator mediator) : ControllerBase
+{
+    [HttpGet("/api/artists/{id:int}", Name = "GetArtistById")]
+    [Tags("Artists")]
+    [EndpointSummary("Get an artist by id")]
+    [EndpointGroupName("general")]
+    [ProducesResponseType(typeof(ArtistDetailResponseModel), StatusCodes.Status200OK)]
+    [ProducesResponseType(typeof(ProblemDetails), StatusCodes.Status404NotFound)]
+    public async Task<IActionResult> GetById(int id, CancellationToken cancellationToken)
+    {
+        Option<ArtistViewModel> result = await mediator.Send(new GetArtistById(id), cancellationToken);
+        return result.Map(vm => Project(id, vm)).ToGetResult();
+    }
+
+    private static ArtistDetailResponseModel Project(int id, ArtistViewModel vm) =>
+        new(
+            id,
+            vm.Name,
+            string.IsNullOrWhiteSpace(vm.Disambiguation) ? null : vm.Disambiguation,
+            string.IsNullOrWhiteSpace(vm.Biography) ? null : vm.Biography,
+            ApiArtwork.Root(vm.Thumbnail, ArtworkKind.Thumbnail),
+            ApiArtwork.Root(vm.FanArt, ArtworkKind.FanArt),
+            vm.Genres,
+            vm.Styles,
+            vm.Moods,
+            vm.Languages.Map(c => c.EnglishName).ToList());
+}