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/Tracks/Responses/TrackApplicationResponse.cs

@@ -0,0 +1,20 @@
+using Musify.Domain.Entities;
+
+namespace Musify.Application.Tracks.Responses
+{
+    public record TrackApplicationResponse(
+        Guid Id,
+        string Title,
+        DateTime CreatedAt,
+        DateTime UpdatedAt)
+    {
+        public static TrackApplicationResponse FromEntity(Track track)
+        {
+            return new TrackApplicationResponse(
+                track.Id,
+                track.Title,
+                track.CreatedAt,
+                track.UpdatedAt);
+        }
+    }
+}