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/Tracks/Handlers/CreateTrackCommandHandler.cs

@@ -0,0 +1,124 @@
+using Ardalis.Result;
+using Mediator;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.Extensions.Logging;
+using Musify.Application.Configuration;
+using Musify.Application.Shared;
+using Musify.Application.Events;
+using Musify.Application.Tracks.Commands;
+using Musify.Application.Tracks.Responses;
+using Musify.Application.Extensions;
+using Musify.Application.Services;
+using Musify.Domain.Entities;
+using Musify.Domain.ValueObjects;
+using Musify.Application.Contracts;
+
+namespace Musify.Application.Tracks.Handlers
+{
+    public class CreateTrackCommandHandler(
+        IDatabase database,
+        IEventBus eventBus,
+        UploadIntentValidator uploadIntentValidator,
+        ILogger<CreateTrackCommandHandler> logger,
+        ApplicationStorageConfiguration storageConfiguration,
+        TrackConfiguration trackConfiguration,
+        UploadIntentConfiguration uploadIntentConfiguration)
+        : ICommandHandler<CreateTrackCommand, Result<TrackApplicationResponse>>
+    {
+        public async ValueTask<Result<TrackApplicationResponse>> Handle(CreateTrackCommand request, CancellationToken cancellationToken)
+        {
+            var userExists = await database.Users.AsNoTracking()
+                .AnyAsync(u => u.Id == request.UserId, cancellationToken);
+            if (!userExists)
+            {
+                logger.LogWarning("User {UserId} not found", request.UserId);
+                return Result.NotFound($"User {request.UserId} not found");
+            }
+
+            var pictureValidation = await uploadIntentValidator.ValidateAndLoadAsync(
+                uploadIntentConfiguration,
+                request.PictureIntentId, request.UserId, cancellationToken);
+            if (!pictureValidation.IsSuccess)
+                return pictureValidation.As<UploadIntent, TrackApplicationResponse>();
+
+            var audioValidation = await uploadIntentValidator.ValidateAndLoadAsync(
+                uploadIntentConfiguration,
+                request.AudioIntentId, request.UserId, cancellationToken);
+            if (!audioValidation.IsSuccess)
+                return audioValidation.As<UploadIntent, TrackApplicationResponse>();
+
+            var pictureIntent = pictureValidation.Value;
+            var audioIntent = audioValidation.Value;
+
+            var finalPictureKey = trackConfiguration.Routes.BuildOriginalPicturePath(request.UserId, pictureIntent.ObjectName);
+            var finalAudioKey = trackConfiguration.Routes.BuildOriginalAudioPath(request.UserId, audioIntent.ObjectName);
+            var audioProcessedFolderKey = trackConfiguration.Routes.BuildProcessedAudioPath(Guid.NewGuid().ToString());
+
+            var trackEntity = new Track
+            {
+                Title = request.Title,
+                NormalizedTitle = request.Title.ToUpperInvariant(),
+                OriginalPictureName = pictureIntent.ObjectName,
+                OriginalAudioName = audioIntent.ObjectName,
+                SmallPictureName = trackConfiguration.Routes.PresetSmallPicture,
+                MediumPictureName = trackConfiguration.Routes.PresetMediumPicture,
+                LargePictureName = trackConfiguration.Routes.PresetLargePicture,
+                PicturesProcessingStatus = ProcessingStatus.Pending,
+                AudioTranscodeProcessingStatus = ProcessingStatus.Pending
+            };
+
+            await database.Tracks.AddAsync(trackEntity, cancellationToken);
+
+            await database.UserHasTracks.AddAsync(new UserHasTrack
+            {
+                UserId = request.UserId,
+                TrackId = trackEntity.Id
+            }, cancellationToken);
+
+            try
+            {
+                await eventBus.PublishAsync(
+                    new CreateTrackResourcesEvent(
+                        trackEntity.Id,
+                        pictureIntent.Id,
+                        audioIntent.Id,
+                        storageConfiguration.Bucket,
+                        pictureIntent.Key,
+                        finalPictureKey,
+                        audioIntent.Key,
+                        finalAudioKey,
+                        audioProcessedFolderKey,
+                        new ImageSize(
+                            trackConfiguration.Routes.SmallPicturesPath,
+                            trackConfiguration.PicturesSizes.SmallPictureWidth,
+                            trackConfiguration.PicturesSizes.SmallPictureHeight),
+                        new ImageSize(
+                            trackConfiguration.Routes.MediumPicturesPath,
+                            trackConfiguration.PicturesSizes.MediumPictureWidth,
+                            trackConfiguration.PicturesSizes.MediumPictureHeight),
+                        new ImageSize(
+                            trackConfiguration.Routes.LargePicturesPath,
+                            trackConfiguration.PicturesSizes.LargePictureWidth,
+                            trackConfiguration.PicturesSizes.LargePictureHeight)),
+                    cancellationToken);
+            }
+            catch (Exception exception)
+            {
+                logger.LogError(exception, "Failed to publish create track event for track {TrackId}", trackEntity.Id);
+                return Result.Error($"Failed to create track for {request.Title}");
+            }
+
+            try
+            {
+                await database.SaveChangesAsync(cancellationToken);
+            }
+            catch (Exception exception)
+            {
+                logger.LogError(exception, "Failed to save track {Title}", request.Title);
+                return Result.Error($"Failed to create track for {request.Title}");
+            }
+
+            return Result.Created(TrackApplicationResponse.FromEntity(trackEntity));
+        }
+    }
+}