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).

Start by tracing the campaign state column, transition function, signup buttons, materialisation, audit_log, campaign_members, and max_sessions references. Done means all listed lifecycle transitions and roster rules work, audit_log records every transition, HIATUS and CONCLUDED control materialisation correctly, and directly RUNNING campaigns use campaign_members.

索引モデルが issue の本文から書いたものです。