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/Commands/RequestPlayListPictureUploadCommand.cs

@@ -0,0 +1,13 @@
+using Ardalis.Result;
+using Mediator;
+using Musify.Application.PlayLists.Responses;
+
+namespace Musify.Application.PlayLists.Commands
+{
+    public record RequestPlayListPictureUploadCommand(
+        long UserId,
+        string FileType,
+        string ContentType,
+        long? ExpectedSizeBytes = null)
+        : ICommand<Result<PlayListPictureUploadResponse>>;
+}