feat(backend): add add email address endpoint

Foxnouns.Backend/Mailables/AddEmailMailable.cs

@@ -0,0 +1,18 @@
+using Coravel.Mailer.Mail;
+
+namespace Foxnouns.Backend.Mailables;
+
+public class AddEmailMailable(Config config, AddEmailMailableView view)
+    : Mailable<AddEmailMailableView>
+{
+    public override void Build()
+    {
+        To(view.To).From(config.EmailAuth.From!).View("~/Views/Mail/AddEmail.cshtml", view);
+    }
+}
+
+public class AddEmailMailableView : BaseView
+{
+    public required string Code { get; init; }
+    public required string Username { get; init; }
+}