refactor: split shared models and DTOs

src/ASTRAIN.Shared/Models/User.cs

@@ -0,0 +1,31 @@
+using System.ComponentModel.DataAnnotations;
+
+namespace ASTRAIN.Shared.Models;
+
+/// <summary>
+/// Represents an application user identified by an 8-character key.
+/// </summary>
+public class User
+{
+    /// <summary>
+    /// Gets or sets the unique 8-character user identifier.
+    /// </summary>
+    [Key]
+    [MaxLength(8)]
+    public string Id { get; set; } = string.Empty;
+
+    /// <summary>
+    /// Gets or sets the creation timestamp in UTC.
+    /// </summary>
+    public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
+
+    /// <summary>
+    /// Gets or sets the exercises owned by this user.
+    /// </summary>
+    public List<Exercise> Exercises { get; set; } = new();
+
+    /// <summary>
+    /// Gets or sets the routines owned by this user.
+    /// </summary>
+    public List<Routine> Routines { get; set; } = new();
+}