Replaces the placeholder Slack-invite handler with a call to the new
PATCH /api/admin/members/:id/slack-status endpoint. Status labels are
reworded to match reality (no Slack API call from this app):
- Pending → Not yet invited
- Invited → Invited <slackInvitedAt>
- Action button copy → 'Mark as Slack invited'
- Removes slackInviteStatus reads from the member detail page (the
remaining repo-wide sweep lands in the cleanup task).
Friendlier tone + ghost emoji on invite/welcome subjects; invite
templates now offer a reply-to-this-email fallback; tighten OIDC
wiki sign-in and event registration confirmation copy.
