test: add tests for smtp mailer config

lib/mv/mailer.ex

@@ -16,4 +16,15 @@ defmodule Mv.Mailer do
   def mail_from do
     Application.get_env(:mv, :mail_from, {"Mila", "noreply@example.com"})
   end
+
+  @doc """
+  Sends a test email to the given address. Used from Global Settings SMTP section.
+
+  Returns `{:ok, email}` on success, `{:error, reason}` on failure (e.g. invalid address,
+  SMTP not configured, connection error). Stub: always returns error until implemented.
+  """
+  @spec send_test_email(String.t()) :: {:ok, Swoosh.Email.t()} | {:error, term()}
+  def send_test_email(_to_email) do
+    {:error, :not_implemented}
+  end
 end