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/AddTrackToPlayListCommandHandler.cs

@@ -0,0 +1,70 @@
+using Ardalis.Result;
+using Mediator;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.Extensions.Logging;
+using Musify.Application.Contracts;
+using Musify.Application.PlayLists.Commands;
+using Musify.Domain.Entities;
+
+namespace Musify.Application.PlayLists.Handlers
+{
+    public class AddTrackToPlayListCommandHandler(
+        IDatabase database,
+        ILogger<AddTrackToPlayListCommandHandler> logger)
+        : ICommandHandler<AddTrackToPlayListCommand, Result>
+    {
+        public async ValueTask<Result> Handle(AddTrackToPlayListCommand request, CancellationToken cancellationToken)
+        {
+            var playList = await database.PlayLists
+                .SingleOrDefaultAsync(pl => pl.Id == request.PlayListId, cancellationToken);
+            if (playList is null)
+            {
+                logger.LogInformation("Playlist {PlayListId} not found", request.PlayListId);
+                return Result.NotFound();
+            }
+
+            if (playList.UserId != request.UserId)
+            {
+                logger.LogWarning("User {UserId} is not the owner of playlist {PlayListId}", request.UserId, request.PlayListId);
+                return Result.Unauthorized();
+            }
+
+            var trackExists = await database.Tracks
+                .AsNoTracking()
+                .AnyAsync(t => t.Id == request.TrackId, cancellationToken);
+            if (!trackExists)
+            {
+                logger.LogInformation("Track {TrackId} not found", request.TrackId);
+                return Result.NotFound($"Track {request.TrackId} not found");
+            }
+
+            var alreadyAdded = await database.PlayListHasTracks
+                .AsNoTracking()
+                .AnyAsync(plt => plt.PlayListId == request.PlayListId && plt.TrackId == request.TrackId, cancellationToken);
+            if (alreadyAdded)
+            {
+                logger.LogInformation("Track {TrackId} already in playlist {PlayListId}", request.TrackId, request.PlayListId);
+                return Result.Conflict("Track is already in the playlist.");
+            }
+
+            var nextPosition = await database.PlayListHasTracks
+                .Where(plt => plt.PlayListId == request.PlayListId)
+                .Select(plt => (int?)plt.Position)
+                .MaxAsync(cancellationToken) + 1 ?? 0;
+
+            await database.PlayListHasTracks.AddAsync(new PlayListHasTrack
+            {
+                PlayListId = request.PlayListId,
+                TrackId = request.TrackId,
+                Position = nextPosition
+            }, cancellationToken);
+
+            await database.SaveChangesAsync(cancellationToken);
+
+            logger.LogInformation("Added track {TrackId} to playlist {PlayListId} at position {Position}",
+                request.TrackId, request.PlayListId, nextPosition);
+
+            return Result.Success();
+        }
+    }
+}