Schema: full game_days, game-day signups, one-off sessions, tables_played

- [ ] `game_days` gains: venue, host, `capacity`, `game_id` (set for `single`), full lifecycle state, channel/message ids, `thread_id`.
- [ ] `signups.target_type = 'game_day'` enabled; `position` for waitlist ordering; the CHECK constraint that a signup never targets a session stays.
- [ ] `sessions.kind = 'one_off'` with `game_day_id` and the CHECK that exactly one of `campaign_id` / `game_day_id` is set — a game day gets a session row so attendance, threads, calendar and Discord events reuse the same machinery.
- [ ] `attendance.tables_played` (optional, nullable) — settles the open question as "yes, optional, recorded by whoever runs the day".