Stamp session_user into archive-trigger snapshots alongside app.user_id

audit-mars-ca condition on MARS-490 (mig 060, per-agent DB roles): archive-trigger snapshots must stamp session_user alongside app.user_id. Until this lands, durable per-agent DML audit cannot be claimed — pg_stat_activity is transient; today's durable attribution is pg logs + archive capture only. DB lane owns.