Jeopardy notices at T-24h

When the answer to "does it run" is no, the response is a date poll, not a cancellation. Time-triggered state changes were the only real casualty of send-only, so they become **new notices**.

- [ ] A `jobs` row at T-24h per session: if quorum is not met, post a jeopardy notice in the thread mentioning the roster, naming who has not answered.
- [ ] The notice carries a *Suggest another day* affordance once date polls exist (phase 4); until then it names the GM.
- [ ] Never mutate the attendance post. The notice is a new message with its own as-of line.