Returns the composite payload needed to render the participant waiting room: session info + leader, AI features flag, attendee roster with computed presence states (present / joining / not_yet_here), reflection state for the orientation bullet, and a server now timestamp clients use for the countdown.

Side effect: upserts the caller's presence heartbeat (session_attendance.last_seen_at, joined_at set on first call).

Gated by live_session_delivery feature flag. Returns 403 when the authenticated user is not enrolled in the session's cohort.