Quorum auto-confirm

The click that crosses the threshold renders the confirmed state — quorum is click-driven, so it fits send-only for free.

- [ ] `campaigns.quorum`; a session is confirmed when the count of `intent = in` reaches it.
- [ ] The click that crosses the threshold rewrites its own message with the confirmed state and posts a short "confirmed" notice in the thread.
- [ ] Dropping back below quorum after confirmation: the click rewrites its message to show it; whether that un-confirms is the organiser's call — surface it, don't decide it.
- [ ] Session state reflects confirmation so `/upcoming` and the console can show it.