fix: join confirmation mail configuration

test/support/failing_mail_adapter.ex

@@ -0,0 +1,10 @@
+defmodule Mv.TestSupport.FailingMailAdapter do
+  @moduledoc """
+  Swoosh adapter that always returns delivery failure. Used in tests to assert
+  that join confirmation email failure is handled (error shown to user, no success UI).
+  """
+  use Swoosh.Adapter
+
+  @impl true
+  def deliver(_email, _config), do: {:error, :forced}
+end