Date poll post: multi-select availability + organiser-only Canonise

Audience: whole server (stage 1). 5 buttons per row means a 10-date poll needs a **multi-select**.

- [ ] Post: candidate dates, current tallies per date, as-of line, a string-select (multi) for availability, **Refresh**, and an organiser-only **Canonise** button.
- [ ] Selecting rewrites the message with new tallies (the click *is* the re-render). Selections are stored in `poll_responses`; the post is never read back.
- [ ] Canonise: organiser-only check via role; applies the poll's `win_rule` to compute the winning date(s), lets the organiser override, and writes `poll_dates.outcome`. Closes the poll.
- [ ] `closes_at` handled by a `jobs` row that posts a "poll closed" notice — never edits the poll post.