Single game day: seats, waitlist, auto-promotion notices

Stage 2 for a single game day: claim 1 of N seats. Audience: whole server.

- [ ] Signup post with **Take a seat / Waitlist / Out** + *Can't make this one — suggest a day* + **Refresh**, as-of line, seat count from `games.max_players`. Each click rewrites its own message.
- [ ] Waitlist ordering by `position`. When a seated player goes Out, promote the head of the waitlist and **post a new promotion notice** mentioning them — never edit the post.
- [ ] Seated players become the roster: attendance stage (In / Out / Maybe, reminders, jeopardy at the game's min players) runs in the day's thread exactly as for a campaign session.