Returns everything the participant active session screen needs in one round trip: session metadata, leader, sections, slide deck, current section index, AI features flag, and the AC #12 commitment trigger.

Authorization: the caller must be an active member of the session's group (group_members row with left_at IS NULL). Otherwise 403.

This endpoint is intended to be polled at <= 2s cadence by the frontend to satisfy AC #6 (participant view updates within 2 seconds of leader action) and IV2.