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.

.gitignore

@@ -11,14 +11,6 @@ artifacts/
 # Rider / VS
 .idea/
 
-# Flutter / Dart
-musify_flutter/.dart_tool/
-musify_flutter/build/
-musify_flutter/.flutter-plugins
-musify_flutter/.flutter-plugins-dependencies
-musify_flutter/**/generated_plugin_registrant.*
-*.g.dart
-
 # OS
 Thumbs.db
 .DS_Store