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.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/Musify.Application/Tracks/Handlers/DeleteTrackCommandHandler.cs

@@ -0,0 +1,70 @@
+using Ardalis.Result;
+using Mediator;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.Extensions.Logging;
+using Musify.Application.Tracks.Commands;
+using Musify.Domain.ValueObjects;
+using Musify.Application.Events;
+using Musify.Application.Contracts;
+
+namespace Musify.Application.Tracks.Handlers
+{
+    public class DeleteTrackCommandHandler(
+        IDatabase database,
+        IEventBus eventBus,
+        ILogger<DeleteTrackCommandHandler> logger)
+        : ICommandHandler<DeleteTrackCommand, Result>
+    {
+        public async ValueTask<Result> Handle(DeleteTrackCommand request, CancellationToken cancellationToken)
+        {
+            var track = await database.Tracks.SingleOrDefaultAsync(t => t.Id == request.TrackId, cancellationToken);
+            if (track is null)
+            {
+                logger.LogInformation("Track {TrackId} not found", request.TrackId);
+                return Result.NotFound();
+            }
+
+            var isOwner = await database.UserHasTracks
+                .AsNoTracking()
+                .AnyAsync(ut => ut.TrackId == request.TrackId && ut.UserId == request.UserId, cancellationToken);
+            if (!isOwner)
+            {
+                logger.LogWarning("User {UserId} unauthorized to delete track {TrackId}", request.UserId, request.TrackId);
+                return Result.Unauthorized();
+            }
+
+            if (track.AudioTranscodeProcessingStatus == ProcessingStatus.Processing
+                || track.AudioTranscodeProcessingStatus == ProcessingStatus.Pending)
+            {
+                logger.LogWarning("Track {TrackId} is currently being processed and cannot be deleted", request.TrackId);
+                return Result.Conflict("Track is currently being processed and cannot be deleted");
+            }
+
+            track.LifeCycleStatus = LifeCycleStatus.Removing;
+
+            try
+            {
+                await eventBus.PublishAsync(
+                    new DeleteTrackEvent(track.Id, request.UserId),
+                    cancellationToken);
+            }
+            catch (Exception exception)
+            {
+                logger.LogError(exception, "Failed to publish delete track event for track {TrackId}", request.TrackId);
+                return Result.Error($"Failed to delete track {request.TrackId}");
+            }
+
+            try
+            {
+                await database.SaveChangesAsync(cancellationToken);
+            }
+            catch (Exception exception)
+            {
+                logger.LogError(exception, "Failed to mark track {TrackId} as processing before deletion", request.TrackId);
+                return Result.Error($"Failed to delete track {request.TrackId}");
+            }
+
+            return Result.NoContent();
+        }
+    }
+}