Campaign lifecycle: FORMING → RUNNING ⇄ HIATUS → CONCLUDED

`FORMING → RUNNING ⇄ HIATUS → CONCLUDED`.

- [ ] State column + transitions as one function; every transition writes `audit_log`.
- [ ] Signup buttons exist only in FORMING. Starting a campaign closes the roster; **a closed roster never reopens on its own.**
- [ ] Started campaigns skip stages 1 and 2 structurally — a campaign entered directly as RUNNING (the four existing ones) gets its roster from `campaign_members`, not from signups.
- [ ] HIATUS pauses materialisation; RUNNING resumes it from the anchor. CONCLUDED stops it for good.
- [ ] `max_sessions` reached → CONCLUDED, or prompt the GM (organiser's call).