chore(91b): clean up dead Blazor residue (Serilog override + stale comment)

Codex-review nits: drop the now-inert "MudBlazor" Serilog level override in
appsettings.json (package removed) and reword the PlaylistController comment
that referenced the deleted Blazor MultiSelectBase.AddItemsToPlaylist path. No
behavior change.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

ErsatzTV/Controllers/Api/PlaylistController.cs

@@ -241,8 +241,8 @@ public class PlaylistController(IMediator mediator) : ControllerBase
         }
 
         // System (generated) playlists must not have items added. The AddItemsToPlaylistHandler
-        // also enforces this (defense-in-depth for the Blazor path, which calls the same command
-        // from MultiSelectBase.AddItemsToPlaylist).
+        // also enforces this (defense-in-depth: the handler is the authoritative guard regardless
+        // of caller).
         Either<BaseError, Unit> result = await mediator.Send(request.ToCommand(id), cancellationToken);
         return result.ToDeletedResult();
     }