refactor: standardize project names to Musify.* and flatten layout

- Flatten nested per-repo folders into flat Musify.* projects at root
- Rename assemblies/csproj: Domain->Musify.Domain, Application->Musify.Application,
  Infrastructure->Musify.Infrastructure, WebApi->Musify.Api, Worker->Musify.Worker,
  StreamingGateway->Musify.StreamingGateway
- Rename WebApi.* namespaces to Musify.Api.*
- Update all ProjectReference paths and the solution file
- Consolidate stray per-repo dotfiles; add root .gitattributes
- Move flutter client out to _archive

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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>();
+    }
+}