Move Student Between Breakouts

Move a student who is ALREADY inside a breakout into a different breakout. Host/admin only.

Unlike assign-latecomer (whose target student sits in the parent room), the student here is inside a child room, so a parent-only broadcast wouldn't reach them. We publish the same breakout_assign message the client already handles into the parent AND every active child, so it reaches the student wherever they currently are; every other student ignores it (the client filters the assignment list by its own user_id).