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/PlayList.cs

@@ -0,0 +1,61 @@
+using System.ComponentModel.DataAnnotations;
+using System.ComponentModel.DataAnnotations.Schema;
+
+using Musify.Domain.Abstractions;
+using Musify.Domain.ValueObjects;
+
+namespace Musify.Domain.Entities
+{
+#pragma warning disable CS8618
+    [Table("PlayLists")]
+    public class PlayList : IAuditable
+    {
+        [Key]
+        public Guid Id { get; set; } = Guid.NewGuid();
+
+        [Required]
+        public required long UserId { get; set; }
+
+        [Required]
+        [MaxLength(50)]
+        public required string Name { get; set; }
+
+        [Required]
+        [MaxLength(50)]
+        public required string NormalizedName { get; set; }
+
+        [Required]
+        [MaxLength(256)]
+        public required string Description { get; set; } = "No description was provided.";
+
+        [Required]
+        [MaxLength(64)]
+        public required string OriginalPictureName { get; set; }
+
+        [Required]
+        [MaxLength(64)]
+        public required string SmallPictureName { get; set; }
+
+        [Required]
+        [MaxLength(64)]
+        public required string MediumPictureName { get; set; }
+
+        [Required]
+        [MaxLength(64)]
+        public required string LargePictureName { get; set; }
+
+        [Required]
+        public LifeCycleStatus LifeCycleStatus { get; set; } = LifeCycleStatus.Active;
+
+        [Required]
+        public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
+
+        [Required]
+        public DateTime UpdatedAt { get; set; } = DateTime.UtcNow;
+
+        [ForeignKey(nameof(UserId)) ]
+        public User User { get; set; }
+
+        public ICollection<PlayListHasTrack> PlayListTracks { get; set; } = new List<PlayListHasTrack>();
+    }
+}