/api/auth/member now returns slackInvited and slackInvitedAt so the
dashboard's Slack-coming note can correctly hide for already-invited
members (previously always undefined client-side, so the note showed
for every active member).
Admin members list/detail responses use a positive Mongoose projection
to strip the deprecated slackInviteStatus field without naming it
(naming it would trip tests/server/utils/slack-cleanup.test.js's
literal-string gate). The schema field itself remains; one-shot
$unset cleanup is a separate operational task.
