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/components/member_filter_component_test.exs

@@ -8,7 +8,10 @@ defmodule MvWeb.Components.MemberFilterComponentTest do
   - Button label and badge logic
   - Filtering to show only boolean custom fields
   """
-  # async: false to prevent PostgreSQL deadlocks when running LiveView tests against DB
+  # Kept async: false. The deferrable-FK migration removed the concurrent
+  # create_member deadlock, but this file additionally showed an async-isolation
+  # failure under load (filtered members from a parallel test leaking in), so it
+  # is not trivially async-safe; resolving that is a separate follow-up.
   use MvWeb.ConnCase, async: false
 
   import Phoenix.LiveViewTest