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/Events/DeleteTrackEvent.cs

@@ -0,0 +1,6 @@
+namespace Musify.Application.Events
+{
+    public record DeleteTrackEvent(
+        Guid TrackId,
+        long UserId);
+}