refactor: review remarks

lib/membership/join_notifier.ex

@@ -0,0 +1,13 @@
+defmodule Mv.Membership.JoinNotifier do
+  @moduledoc """
+  Behaviour for sending join-related emails (confirmation, already member, already pending).
+
+  The domain calls this module instead of MvWeb.Emails directly, so the domain layer
+  does not depend on the web layer. The default implementation is set in config
+  (`config :mv, :join_notifier, MvWeb.JoinNotifierImpl`). Tests can override with a mock.
+  """
+  @callback send_confirmation(email :: String.t(), token :: String.t(), opts :: keyword()) ::
+              {:ok, term()} | {:error, term()}
+  @callback send_already_member(email :: String.t()) :: {:ok, term()} | {:error, term()}
+  @callback send_already_pending(email :: String.t()) :: {:ok, term()} | {:error, term()}
+end