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/GetPlayListByIdQueryHandler.cs

@@ -0,0 +1,26 @@
+using Ardalis.Result;
+using Microsoft.EntityFrameworkCore;
+using Musify.Application.PlayLists.Responses;
+using Musify.Application.PlayLists.Queries;
+using Mediator;
+using Musify.Application.Contracts;
+
+namespace Musify.Application.PlayLists.Handlers
+{
+    public class GetPlayListByIdQueryHandler(IDatabase database)
+        : IQueryHandler<GetPlayListByIdQuery, Result<PlayListApplicationResponse>>
+    {
+        public async ValueTask<Result<PlayListApplicationResponse>> Handle(GetPlayListByIdQuery request, CancellationToken cancellationToken)
+        {
+            var playList = await database.PlayLists
+                .AsNoTracking()
+                .Where(p => p.Id == request.Id)
+                .Select(p => PlayListApplicationResponse.FromEntity(p))
+                .SingleOrDefaultAsync(cancellationToken);
+
+            return playList is null
+                ? Result<PlayListApplicationResponse>.NotFound()
+                : Result.Success(playList);
+        }
+    }
+}