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/Abstractions/IAuditable.cs

@@ -0,0 +1,9 @@
+namespace Musify.Domain.Abstractions
+{
+    public interface IAuditable
+    {
+        DateTime CreatedAt { get; set; }
+
+        DateTime UpdatedAt { get; set; }
+    }
+}

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>();
+    }
+}

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; }
+    }
+}

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;
+    }
+}

backend/Musify.Domain/Entities/Upload.cs

@@ -0,0 +1,29 @@
+using System.ComponentModel.DataAnnotations;
+using System.ComponentModel.DataAnnotations.Schema;
+
+using Musify.Domain.ValueObjects;
+
+namespace Musify.Domain.Entities
+{
+    [Table("Upload")]
+    public class Upload
+    {
+        [Key]
+        public Guid Id { get; set; } = Guid.NewGuid();
+
+        [Required]
+        public required string Bucket { get; set; }
+
+        [Required]
+        public required string Key { get; set; }
+
+        [Required]
+        public required string ContentType { get; set; }
+
+        [Required]
+        public DateTime CreatedTime { get; set; } = DateTime.UtcNow;
+
+        [Required]
+        public UploadState State { get; set; } = UploadState.Pending;
+    }
+}

backend/Musify.Domain/Entities/UploadIntent.cs

@@ -0,0 +1,46 @@
+using System.ComponentModel.DataAnnotations;
+using System.ComponentModel.DataAnnotations.Schema;
+
+using Musify.Domain.ValueObjects;
+
+namespace Musify.Domain.Entities
+{
+    [Table("UploadIntents")]
+    public class UploadIntent
+    {
+        [Key]
+        public Guid Id { get; set; } = Guid.NewGuid();
+
+        [Required]
+        public required long UserId { get; set; }
+
+        [Required, MaxLength(128)]
+        public required string Bucket { get; set; }
+
+        [Required, MaxLength(512)]
+        public required string Key { get; set; }
+
+        [Required, MaxLength(128)]
+        public required string ObjectName { get; set; }
+
+        [Required, MaxLength(128)]
+        public required string ContentType { get; set; }
+
+        public long? ExpectedSizeBytes { get; set; }
+
+        [Required]
+        public UploadIntentPurpose Purpose { get; set; }
+
+        [Required]
+        public UploadIntentStatus Status { get; set; } = UploadIntentStatus.Issued;
+
+        [Required]
+        public DateTime ExpiresAt { get; set; }
+
+        [Required]
+        public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
+
+        [ForeignKey(nameof(UserId)) ]
+        public User User { get; set; } = null!;
+    }
+}

backend/Musify.Domain/Entities/User.cs

@@ -0,0 +1,41 @@
+using System.ComponentModel.DataAnnotations;
+using System.ComponentModel.DataAnnotations.Schema;
+using Musify.Domain.Abstractions;
+
+namespace Musify.Domain.Entities
+{
+    [Table("User")]
+    public class User : IAuditable
+    {
+        [Key]
+        [DatabaseGenerated(DatabaseGeneratedOption.None)]
+        public required long Id { get; set; }
+
+        [Required]
+        [MaxLength(48)]
+        public required string Name { get; set; }
+
+        [Required]
+        [MaxLength(48)]
+        public required string NormalizedName { get; set; }
+
+        [MaxLength(48)]
+        public string? FirstName { get; set; }
+
+        [MaxLength(48)]
+        public string? SecondName { get; set; }
+
+        [MaxLength(2048)]
+        public string? ProfilePictureUrl { 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<PlayList> PlayLists { get; set; } = new List<PlayList>();
+    }
+}

backend/Musify.Domain/Entities/UserHasTrack.cs

@@ -0,0 +1,24 @@
+using System.ComponentModel.DataAnnotations;
+using System.ComponentModel.DataAnnotations.Schema;
+
+namespace Musify.Domain.Entities
+{
+#pragma warning disable CS8618
+    [Table("UserHasTrack")]
+    public class UserHasTrack
+    {
+        public Guid Id { get; set; } = Guid.NewGuid();
+
+        [Required]
+        public required long UserId { get; set; }
+
+        [Required]
+        public required Guid TrackId { get; set; }
+
+        [ForeignKey(nameof(UserId)) ]
+        public User User { get; set; }
+
+        [ForeignKey(nameof(TrackId)) ]
+        public Track Track { get; set; }
+    }
+}

backend/Musify.Domain/Musify.Domain.csproj

@@ -0,0 +1,3 @@
+<Project Sdk="Microsoft.NET.Sdk">
+
+</Project>

backend/Musify.Domain/ValueObjects/LifeCycleStatus.cs

@@ -0,0 +1,8 @@
+namespace Musify.Domain.ValueObjects
+{
+    public enum LifeCycleStatus
+    {
+        Active = 0,
+        Removing = 1
+    }
+}

backend/Musify.Domain/ValueObjects/ProcessingStatus.cs

@@ -0,0 +1,10 @@
+namespace Musify.Domain.ValueObjects
+{
+    public enum ProcessingStatus
+    {
+        Pending = 0,
+        Processing = 1,
+        Completed = 2,
+        Failed = 3
+    }
+}

backend/Musify.Domain/ValueObjects/UploadIntentPurpose.cs

@@ -0,0 +1,9 @@
+namespace Musify.Domain.ValueObjects
+{
+    public enum UploadIntentPurpose
+    {
+        PlayListPicture,
+        TrackPicture,
+        TrackAudio,
+    }
+}

backend/Musify.Domain/ValueObjects/UploadIntentStatus.cs

@@ -0,0 +1,9 @@
+namespace Musify.Domain.ValueObjects
+{
+    public enum UploadIntentStatus
+    {
+        Issued,
+        Consumed,
+        Expired,
+    }
+}

backend/Musify.Domain/ValueObjects/UploadState.cs

@@ -0,0 +1,9 @@
+namespace Musify.Domain.ValueObjects
+{
+    public enum UploadState
+    {
+        Pending,
+        Failed,
+        Successful,
+    }
+}