Create Announcement

Staff broadcast. Body: {title?, body, group_ids?: [..], important?} — no group_ids means org-wide; several ids fan out one announcement per section (Classroom multi-class 'For' pattern). important is the only delivery control a teacher gets, and it is about the MESSAGE, not the channel: it reaches students on the daily digest right away.

The audience is authorized by _require_announcement_scope, not by _require_staff_role: "is a teacher somewhere in this school" was standing in for "may address these people", which is the same substitution _require_group_staff was written to end on the section endpoints.