chore: group .NET into backend/ and drop non-.NET test artifacts

- Move all .NET (6 projects, solution, Directory.*.props, nuget.config)
  into backend/ so the monorepo root can host non-.NET projects later
- Remove throwaway test artifacts: player.html, pl.json
- Trim dead Flutter rules from .gitignore

Build from backend/Musify.slnx: 0 warnings, 0 errors.

backend/Musify.Application/PlayLists/Handlers/DeletePlayListCommandHandler.cs

@@ -0,0 +1,60 @@
+using Ardalis.Result;
+using Mediator;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.Extensions.Logging;
+using Musify.Application.Contracts;
+using Musify.Application.Events;
+using Musify.Application.PlayLists.Commands;
+using Musify.Domain.ValueObjects;
+
+namespace Musify.Application.PlayLists.Handlers
+{
+    public class DeletePlayListCommandHandler(
+        IDatabase database,
+        IEventBus eventBus,
+        ILogger<DeletePlayListCommandHandler> logger)
+        : ICommandHandler<DeletePlayListCommand, Result>
+    {
+        public async ValueTask<Result> Handle(DeletePlayListCommand request, CancellationToken cancellationToken)
+        {
+            var playList = await database.PlayLists.SingleOrDefaultAsync(p => p.Id == request.PlayListId, cancellationToken);
+            if (playList is null)
+            {
+                logger.LogDebug("Playlist {PlayListId} not found", request.PlayListId);
+                return Result.NotFound();
+            }
+
+            if (playList.UserId != request.UserId)
+            {
+                logger.LogWarning("Playlist {PlayListId} does not belong to user {UserId}", request.PlayListId, request.UserId);
+                return Result.Unauthorized();
+            }
+
+            playList.LifeCycleStatus = LifeCycleStatus.Removing;
+
+            try
+            {
+                await eventBus.PublishAsync(
+                    new DeletePlayListEvent(playList.Id, request.UserId),
+                    cancellationToken);
+            }
+            catch (Exception exception)
+            {
+                logger.LogError(exception, "Failed to publish delete playlist event for playlist {PlayListId}", request.PlayListId);
+                return Result.Error($"Failed to delete playlist {request.PlayListId}");
+            }
+
+            try
+            {
+                await database.SaveChangesAsync(cancellationToken);
+            }
+            catch (Exception exception)
+            {
+                logger.LogError(exception, "Failed to mark playlist {PlayListId} as removing", request.PlayListId);
+                return Result.Error($"Failed to delete playlist {request.PlayListId}");
+            }
+
+            return Result.NoContent();
+        }
+    }
+}