Recurrence as anchor + interval_weeks; hourly horizon materialiser

Discord can't own recurrence (weekly = exactly one weekday; `count`/`end` can't be set externally), so Orrey does. Carried over from Hermuz as an idea: **recurrence as anchor + interval_weeks, anchor possibly in the past.**

- [ ] Next-N-occurrences from `recurrence_anchor` + `interval_weeks`, skipping occurrences before now, numbering from `first_session_number`.
- [ ] Hourly cron (`0 * * * *`): materialise sessions out to the horizon for every RUNNING campaign; enqueue Discord-event and Google projections for each new one.
- [ ] Horizon for **Discord scheduled events is capped at two upcoming per campaign**, treated as disposable (100 events/guild). Google and D1 can run further ahead — horizon size in `settings`.
- [ ] Idempotent: re-running materialisation creates nothing twice.
- [ ] Schedule the attendance post as a `jobs` row at the right lead time rather than posting at materialisation.

Start by tracing the existing campaign/session scheduling paths and where RUNNING campaigns, settings, D1 rows, Discord projections, Google projections, and jobs rows are handled. The issue is done when recurrence_anchor plus interval_weeks can materialise future sessions idempotently on an hourly cron, cap Discord events at two upcoming per campaign, and schedule attendance posts as jobs.

Do mô hình lập chỉ mục viết ra từ nội dung của issue.