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/GetPlayListTracksQueryHandler.cs

@@ -0,0 +1,38 @@
+using Ardalis.Result;
+using Mediator;
+using Microsoft.EntityFrameworkCore;
+using Musify.Application.Contracts;
+using Musify.Application.Pagination;
+using Musify.Application.PlayLists.Queries;
+using Musify.Application.Tracks.Responses;
+using X.PagedList.EF;
+
+namespace Musify.Application.PlayLists.Handlers
+{
+    public class GetPlayListTracksQueryHandler(IDatabase database)
+        : IQueryHandler<GetPlayListTracksQuery, Result<PaginatedResponse<TrackApplicationResponse>> >
+    {
+        public async ValueTask<Result<PaginatedResponse<TrackApplicationResponse> >> Handle(GetPlayListTracksQuery request, CancellationToken cancellationToken)
+        {
+            var playListExists = await database.PlayLists
+                .AsNoTracking()
+                .AnyAsync(p => p.Id == request.PlayListId, cancellationToken);
+            if (!playListExists)
+                return Result<PaginatedResponse<TrackApplicationResponse>>.NotFound();
+
+            var tracksQuery = database.PlayListHasTracks
+                .AsNoTracking()
+                .Include(plt => plt.Track)
+                .Where(plt => plt.PlayListId == request.PlayListId);
+
+            var totalCount = await tracksQuery.CountAsync(cancellationToken);
+
+            var pagedTracks = await tracksQuery
+                .OrderBy(plt => plt.Position)
+                .Select(plt => TrackApplicationResponse.FromEntity(plt.Track))
+                .ToPagedListAsync(request.PageNumber, request.PageSize, totalCount, cancellationToken);
+
+            return Result.Success(PaginatedResponse<TrackApplicationResponse>.FromPagedList(pagedTracks));
+        }
+    }
+}