Endpoint that flips a member's slackInvited flag manually after the
admin has actually sent the Slack invitation through Slack's UI. No
Slack API call is made from this app.
- Body validated via Zod literal-true schema (no undo path for the
pilot — admins correct mistakes in the database if needed).
- Idempotent: re-marking an already-invited member is a no-op,
preserving the original slackInvitedAt and not duplicating the
activity log entry.
- Activity log: slack_invited_manually, actor = admin from
requireAdmin, subject = the target member.
