fix(api): root library-browse artwork URLs and add season drill-in (#180)

Part of the #180 library picker fixes (API side).

Broken artwork: GetLibraryBrowseItemsHandler.Artwork returned Blazor-convention
values (a bare cache filename, or a RELATIVE jellyfin/emby proxy path). Blazor's
GetPosterUrl prefixes those with "artwork/posters/" against <base href="/">, but
the React SPA renders item.artwork raw as <img src> from under /app/, so every
image 404'd. The handler now returns rooted, directly-usable URLs
(/artwork/posters/... or /artwork/thumbnails/... per ArtworkKind; jellyfin/emby
mapped to their proxy routes with fillHeight/maxHeight; absolute http(s) URLs
passed through; empty stays empty).

Seasons with no poster of their own now fall back to the parent show's poster
(SeasonArtwork + the extra ShowMetadata.Artwork include).

Season drill-in: GET /api/library/browse gains an optional `parentId` query
param that, with mediaType=TelevisionSeason, returns that show's seasons
(season-number order, bypassing Lucene) so the SPA can expand a show into its
seasons. OpenAPI spec regenerated.

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

ErsatzTV/Controllers/Api/LibraryBrowseController.cs

@@ -22,12 +22,13 @@ public class LibraryBrowseController(IMediator mediator) : ControllerBase
         [FromQuery] LibraryBrowseMediaType? mediaType = null,
         [FromQuery] int pageNum = 0,
         [FromQuery] int pageSize = 100,
+        [FromQuery] int? parentId = null,
         CancellationToken cancellationToken = default)
     {
         int clampedPageNum = Math.Max(0, pageNum);
         int clampedPageSize = Math.Clamp(pageSize, 1, MaxPageSize);
         return await mediator.Send(
-            new GetLibraryBrowseItems(query, libraryId, mediaType, clampedPageNum, clampedPageSize),
+            new GetLibraryBrowseItems(query, libraryId, mediaType, clampedPageNum, clampedPageSize, parentId),
             cancellationToken);
     }
 }

ErsatzTV/wwwroot/openapi/v1.json

@@ -4968,6 +4968,14 @@
               "format": "int32",
               "default": 100
             }
+          },
+          {
+            "name": "parentId",
+            "in": "query",
+            "schema": {
+              "type": "integer",
+              "format": "int32"
+            }
           }
         ],
         "responses": {