test(member-live): keep deadlock-prone member tests synchronous

These member/group/custom-field LiveView tests stay async: false. With the
foreign keys now deferrable the create_member deadlock no longer forces it, so
the rationale is updated: they remain synchronous as a deferred scope decision,
and index_groups_url_params/member_filter_component additionally have separate
async-isolation issues that must be fixed before they can run in parallel.

test/mv_web/member_live/index_groups_accessibility_test.exs

@@ -8,7 +8,10 @@ defmodule MvWeb.MemberLive.IndexGroupsAccessibilityTest do
   - Sort header has aria-label for screen reader
   - Keyboard navigation works (Tab through filter, sort header)
   """
-  # async: false to prevent PostgreSQL deadlocks when creating members and groups
+  # Kept async: false as a deferred scope decision. The deferrable-FK migration
+  # removed the concurrent-create_member deadlock that previously forced this, so
+  # re-flipping this members/groups suite to async is a possible follow-up rather
+  # than part of the original change.
   use MvWeb.ConnCase, async: false
   import Phoenix.LiveViewTest
   require Ash.Query