feat: Add Register DTOs and SP for public registration (Refs #2)

Backend/PruebaGentle.Core/DTOs/RegisterResponseDto.cs

@@ -0,0 +1,8 @@
+namespace PruebaGentle.Core.DTOs;
+
+public class RegisterResponseDto
+{
+    public string Token { get; set; } = string.Empty;
+    public DateTime ExpiresAt { get; set; }
+    public int UserId { get; set; }
+}