refactor: address review comments for join request settings

test/membership/setting_join_form_test.exs

@@ -11,6 +11,8 @@ defmodule Mv.Membership.SettingJoinFormTest do
   - `Mv.Membership.get_join_form_allowlist/0` is implemented and returns the allowlist
     for the public join form (subtask 4).
   """
+  # Settings is a singleton; tests mutate shared DB state. We use async: false and on_exit to restore
+  # original values because Ecto Sandbox transaction rollback does not apply to this singleton pattern.
   use Mv.DataCase, async: false
 
   alias Mv.Constants