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/Responses/TrackUploadUrlsResponse.cs

@@ -0,0 +1,16 @@
+namespace Musify.Application.Tracks.Responses
+{
+    public record TrackUploadUrlsResponse(
+        Guid PictureIntentId,
+        Guid AudioIntentId,
+        string Bucket,
+        string PictureKey,
+        string PictureName,
+        string PictureContentType,
+        string PictureUploadUrl,
+        string AudioKey,
+        string AudioName,
+        string AudioContentType,
+        string AudioUploadUrl,
+        int ExpiresInSeconds);
+}