Add new Backend structure with proper .NET 8 projects

src/Backend/DiunaBI.Core/Models/User.cs

@@ -0,0 +1,17 @@
+using System;
+using System.ComponentModel.DataAnnotations;
+
+namespace DiunaBI.Core.Models;
+
+public class User
+{
+    #region Properties
+    [Key]
+    public Guid Id { get; init; }
+    [StringLength(50)]
+    public string? Email { get; init; }
+    [StringLength(50)]
+    public string? UserName { get; init; }
+    public DateTime CreatedAt { get; init; }
+    #endregion
+}