Reminder ladder with DM fallback (error 50007)

- [ ] A ladder of `jobs` rows per session (e.g. T-72h, T-24h, T-2h — steps in `settings`) that nudge people with `intent` null or maybe.
- [ ] First try a DM. Bot DMs can't be pre-checked — error **`50007`** is how you learn. On `50007` set `users.dm_state = closed`, treat it as permanent, and fall back to a mention in the session thread.
- [ ] Respect `dm_state = closed` on later reminders without retrying the DM.
- [ ] Each reminder is a new message; none edits anything.