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.Domain/Entities/PlayListHasTrack.cs

@@ -0,0 +1,30 @@
+using System.ComponentModel.DataAnnotations;
+using System.ComponentModel.DataAnnotations.Schema;
+
+namespace Musify.Domain.Entities
+{
+#pragma warning disable CS8618
+    [Table("PlayListHasTrack")]
+    public class PlayListHasTrack
+    {
+        public Guid Id { get; set; } = Guid.NewGuid();
+
+        [Required]
+        public required Guid PlayListId { get; set; }
+
+        [Required]
+        public required Guid TrackId { get; set; }
+
+        [Required]
+        public int Position { get; set; } = 0;
+
+        [Required]
+        public DateTime AddedAt { get; set; } = DateTime.UtcNow;
+
+        [ForeignKey(nameof(PlayListId)) ]
+        public PlayList PlayList { get; set; }
+
+        [ForeignKey(nameof(TrackId)) ]
+        public Track Track { get; set; }
+    }
+}