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.

Start by tracing the click-driven quorum flow around `campaigns.quorum` and the session state used by `/upcoming` and the console. Verify the threshold-crossing message rewrite, the thread's short "confirmed" notice, and the below-quorum state are all surfaced without deciding whether confirmation is undone.

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