test: add tests for join mail confirmation

lib/mv_web/router.ex

@@ -126,6 +126,9 @@ defmodule MvWeb.Router do
       overrides: [MvWeb.AuthOverrides, AshAuthentication.Phoenix.Overrides.DaisyUI],
       gettext_backend: {MvWeb.Gettext, "auth"}
 
+    # Public join confirmation (double opt-in); /confirm* is already public in CheckPagePermission
+    get "/confirm_join/:token", JoinConfirmController, :confirm
+
     # Remove this if you do not use the magic link strategy.
     # magic_sign_in_route(Mv.Accounts.User, :magic_link,
     #   auth_routes_prefix: "/auth",