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.
Replace the fixed Process.sleep waits in the import, members-PDF and
field-visibility tests with event-based / bounded-poll waits on the
observable condition, removing a known flakiness vector.
Consider visible custom fields in compute_final_field_selection so that
a link with only custom_field_X is not wrongly treated as invalid and
reverted to session/global. Add test for URL containing only custom field.
