At 20:25-20:39Z the NFS storage under steward_db slowed badly (checkpoint sync 4.6 s, then 70.7 s with one file at 21.9 s, against the usual 0.002 s). Ingest commits stalled past the agent's 10 s timeout and were dropped, and last_seen_at, written in that same transaction, went with them. Agents posting every few seconds read as silent for 3 minutes, and "Agent offline" fired and emailed for all three hosts. Ingest now notes each agent's arrival in memory right after the (read-only) registration lookup, before any write; seconds_since_seen counts from the later of last_seen_at and that arrival. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>