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.

backend/Musify.Domain/Entities/Track.cs

@@ -0,0 +1,78 @@
+using System.ComponentModel.DataAnnotations;
+using System.ComponentModel.DataAnnotations.Schema;
+using System.Diagnostics.CodeAnalysis;
+
+using Musify.Domain.Abstractions;
+using Musify.Domain.ValueObjects;
+
+namespace Musify.Domain.Entities
+{
+#pragma warning disable CS8618
+    public class Track : IAuditable
+    {
+        [Key]
+        public Guid Id { get; set; } = Guid.NewGuid();
+
+        [Required]
+        [MaxLength(50)]
+        public required string Title { get; set; }
+
+        [Required]
+        [MaxLength(50)]
+        public required string NormalizedTitle { get; set; }
+
+        [Required]
+        [MaxLength(64)]
+        public required string OriginalPictureName { get; set; }
+
+        [MaxLength(64)]
+        public string? SmallPictureName { get; set; }
+
+        [MaxLength(64)]
+        public string? MediumPictureName { get; set; }
+
+        [MaxLength(64)]
+        public string? LargePictureName { get; set; }
+
+        [Required]
+        [MaxLength(64)]
+        public required string OriginalAudioName { get; set; }
+
+        [MaxLength(64)]
+        public string? AudioFolderName { get; set; }
+
+        [Required]
+        public ProcessingStatus PicturesProcessingStatus { get; set; } = ProcessingStatus.Pending;
+
+        [Required]
+        public ProcessingStatus AudioTranscodeProcessingStatus { get; set; } = ProcessingStatus.Pending;
+
+        [Required]
+        public LifeCycleStatus LifeCycleStatus { get; set; } = LifeCycleStatus.Active;
+
+        [Required]
+        public int RetryCount { get; set; } = 0;
+
+        public DateTime? LastRetryAt { get; set; }
+
+        [Required]
+        public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
+
+        [Required]
+        public DateTime UpdatedAt { get; set; } = DateTime.UtcNow;
+
+        public ICollection<UserHasTrack> UserTracks { get; set; } = new List<UserHasTrack>();
+
+        public ICollection<PlayListHasTrack> PlayListTracks { get; set; } = new List<PlayListHasTrack>();
+
+        [NotMapped]
+        [MemberNotNullWhen(true, nameof(SmallPictureName)) ]
+        [MemberNotNullWhen(true, nameof(MediumPictureName)) ]
+        [MemberNotNullWhen(true, nameof(LargePictureName)) ]
+        public bool IsPicturesProcessed => PicturesProcessingStatus == ProcessingStatus.Completed;
+
+        [NotMapped]
+        [MemberNotNullWhen(true, nameof(AudioFolderName)) ]
+        public bool IsAudioProcessed => AudioTranscodeProcessingStatus == ProcessingStatus.Completed;
+    }
+}