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/GetTracksByUserIdQueryHandler.cs

@@ -0,0 +1,41 @@
+using Ardalis.Result;
+using Mediator;
+using Microsoft.EntityFrameworkCore;
+using Musify.Application.Pagination;
+using Musify.Application.Tracks.Queries;
+using Musify.Application.Tracks.Responses;
+using X.PagedList.EF;
+using Musify.Application.Contracts;
+
+namespace Musify.Application.Tracks.Handlers
+{
+    public class GetTracksByUserIdQueryHandler(IDatabase database)
+        : IQueryHandler<GetTracksByUserIdQuery, Result<PaginatedResponse<TrackApplicationResponse>> >
+    {
+        public async ValueTask<Result<PaginatedResponse<TrackApplicationResponse> >> Handle(GetTracksByUserIdQuery request, CancellationToken cancellationToken)
+        {
+            var pageNumber = request.PageNumber < 1 ? 1 : request.PageNumber;
+            var pageSize = request.PageSize < 1 ? 10 : request.PageSize;
+
+            var tracksQuery = database.UserHasTracks
+                .AsNoTracking()
+                .Include(ut => ut.Track)
+                .Where(p => p.UserId == request.UserId);
+
+            if (request.Name is not null)
+            {
+                var normalizedName = request.Name.Trim().ToUpperInvariant();
+                tracksQuery = tracksQuery.Where(t => t.Track.NormalizedTitle.Contains(normalizedName));
+            }
+
+            var totalCount = await tracksQuery.CountAsync(cancellationToken);
+
+            var pagedTracks = await tracksQuery
+                .OrderBy(t => t.Track.CreatedAt)
+                .Select(t => TrackApplicationResponse.FromEntity(t.Track))
+                .ToPagedListAsync(pageNumber, pageSize, totalCount, cancellationToken);
+
+            return Result.Success(PaginatedResponse<TrackApplicationResponse>.FromPagedList(pagedTracks));
+        }
+    }
+}