Create Org Channel

Open a named channel. Body: {name, group_id?, description?, member_ids?}.

group_id absent/null = school-wide (every member sees it); set = scoped to that section (its students, plus school staff). member_ids non-empty = a hand-picked channel for exactly those people (plus the creator); it ignores group_id, and ANY staff role may open one — the audience is chosen, so there is no scope to guard.