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.

A STUDENT with member_ids opens a student group instead (app.services.student_groups, which states the rules): group_id is then one of the student's own sections (optional when they have only one), and every id must be a student or teacher of that section, else 404. The creator is always current_user; no id in the body is ever the caller.