CustomField policies: actor required, no system-actor fallback, error handling

- list_required_custom_fields: require actor (two clauses, no default)
- Member validation: use context.actor only, differentiate Forbidden vs transient errors
- stream_custom_fields: log + send flash on error instead of returning []
- GlobalSettingsLive: handle_info for custom_fields_load_error, put_flash
- Seeds: use Membership.update_member with actor, format

lib/mv_web/live/global_settings_live.ex

@@ -504,6 +504,15 @@ defmodule MvWeb.GlobalSettingsLive do
     {:noreply, put_flash(socket, :error, gettext("Slug does not match. Deletion cancelled."))}
   end
 
+  def handle_info({:custom_fields_load_error, _error}, socket) do
+    {:noreply,
+     put_flash(
+       socket,
+       :error,
+       gettext("Could not load data fields. Please check your permissions.")
+     )}
+  end
+
   @impl true
   def handle_info({:editing_section_changed, section}, socket) do
     {:noreply, assign(socket, :active_editing_section, section)}