feat: add DTOs for basic endpoints

DTOs/ErrorDTO.cs

@@ -0,0 +1,13 @@
+using System.Text.Json.Serialization;
+using System.Xml.Serialization;
+
+namespace Shadow.DTOs;
+
+public class ErrorDTO
+{
+	[JsonPropertyName("code")]
+	public int code { get; set; } = 0;
+
+	[JsonPropertyName("message")]
+	public string message { get; set; } = "Generic error";
+}