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/PlayLists/Handlers/GetPlayListsQueryHandler.cs

@@ -0,0 +1,28 @@
+using Ardalis.Result;
+using Microsoft.EntityFrameworkCore;
+using Musify.Application.Pagination;
+using Musify.Application.PlayLists.Responses;
+using Musify.Application.PlayLists.Queries;
+using X.PagedList.EF;
+using Mediator;
+using Musify.Application.Contracts;
+
+namespace Musify.Application.PlayLists.Handlers
+{
+    public class GetPlayListsQueryHandler(IDatabase database)
+        : IQueryHandler<GetPlayListsQuery, Result<PaginatedResponse<PlayListApplicationResponse> >>
+    {
+        public async ValueTask<Result<PaginatedResponse<PlayListApplicationResponse> >> Handle(GetPlayListsQuery request, CancellationToken cancellationToken)
+        {
+            var totalCount = await database.PlayLists.CountAsync(cancellationToken);
+
+            var pagedPlayLists = await database.PlayLists
+                .AsNoTracking()
+                .OrderBy(p => p.Id)
+                .Select(p => PlayListApplicationResponse.FromEntity(p))
+                .ToPagedListAsync(request.PageNumber, request.PageSize, totalCount, cancellationToken);
+
+            return Result.Success(PaginatedResponse<PlayListApplicationResponse>.FromPagedList(pagedPlayLists));
+        }
+    }
+}