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/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;
+    }
+}