Record the peer leader's waiting-room presence heartbeat (MED-1126)

Stamps the peer leader's session_attendance presence heartbeat (last_seen_at, and joined_at on first contact) so the leader shows as present in the participant waiting room while prepping. Peer leaders never enter the waiting room (prep → Start → active), so the prep page pings this on the same ~4s cadence the waiting room polls. Peer-led sessions only; caller must be the designated session leader.