Get Class Private Messages

The quiet lane, as this caller is allowed to see it. Oldest first.

This is the ONLY delivery path — clients poll it. A teacher's count of waiting questions goes up when they ask for it, never when a student sends one, which is precisely the property Sinead asked for.

WHAT EACH CALLER GETS:

• teaching staff, and anyone who may read the class report: every private thread in the class (the parent room and all of its breakouts).
• a student: their own thread only — what they asked, and what staff replied to them.

since_ms makes the poll cheap: pass the newest ts you already hold. Read access does NOT require being in the room right now, so a coordinator can audit the lane a week later; sending does (_require_room_participant).