Game-day lifecycle: PROPOSED → SEATING → LOCKED → PLAYED / CANCELLED

`PROPOSED → SEATING → LOCKED → PLAYED` / `CANCELLED`.

- [ ] Transitions as one function; PROPOSED → SEATING posts the signup post; SEATING → LOCKED closes signups (organiser action or a `jobs` row at a lead time); LOCKED → PLAYED at `ends_at` via the same attendance auto-assume job as campaign sessions.
- [ ] Each game day projects to a Discord scheduled event and a Google event through the existing projectors.
- [ ] CANCELLED posts a notice and deletes the Discord event; it never edits the signup post.

Start by tracing the existing campaign-session attendance auto-assume job and the existing Discord and Google projectors. Map each requested game-day transition and its side effects, then verify that all listed transitions, event projections, signup behavior, cancellation notice, and scheduled jobs work as specified.

由索引模型根据 Issue 内容生成。