Attendance auto-assume at session end + per-person correction toggles

Attendance: auto-assume from intent at session end; organiser corrects via per-person toggles.

- [ ] A `jobs` row at `ends_at`: for each roster member set `attended` from `intent` (in → 1, out/null → 0, maybe → organiser's call, default 0) with `attended_source = auto`. Session → PLAYED.
- [ ] Post a correction post in the thread: one toggle per person. Organiser-only. Each click flips `attended`, sets `attended_source = gm`, and rewrites its own message.
- [ ] Store the `attended` numbers so flake memory and stats can read them.

Start by locating the session-end job, roster attendance storage, and thread-post interaction mentioned in the issue. Trace how session state, intent, and stats are currently represented. Done means the end-of-session transition records automatic attendance, an organiser-only correction post provides one toggle per person, and stored attendance values are available to flake memory and stats.

索引モデルが issue の本文から書いたものです。