Targeted polls: /reschedule, Suggest another day, and moving a session

The targeted use: move a session. Players propose, GM confirms and triggers.

- [ ] `/reschedule` with `APPLICATION_COMMAND_AUTOCOMPLETE` naming an upcoming session across the caller's campaigns; opens a modal to propose up to 10 dates; creates a targeted poll (refusing if one is already open for that session).
- [ ] *Suggest another day* button on the attendance post and the jeopardy notice does the same thing from the message it sits on.
- [ ] Canonising a targeted poll **moves the session**: update `starts_at`/`ends_at`, post a reschedule notice in the session thread (new message), re-project to Google (update in place — same base32hex id).
- [ ] Discord: if the old scheduled event has lapsed (`COMPLETED`/`CANCELED` are terminal and auto-fire), **mint a new Discord event** and store the new id; otherwise modify.
- [ ] Carry available players over: everyone who marked the winning date available gets `intent = in` on the moved session; a fresh attendance post goes in the thread.

Start from the `/reschedule` command, the attendance post, and the jeopardy notice, then trace how targeted polls are created and canonised. Done means moving the session, posting the reschedule notice and fresh attendance post, carrying available players over, updating or recreating the Discord event as required, and re-projecting to Google with the same base32hex id when possible.

索引モデルが issue の本文から書いたものです。