Adds auto-logging when total /api/session latency exceeds 2s, so we don't
need HERMES_DEBUG_SLOW env var to diagnose regressions in production.
Env var still forces logging on every request for development.
Baseline data on the Chromebook (Celeron N3350, eMMC storage):
- long_session_idle (2,445 msgs) p50=5.8s, p95=9.1s, max=9.7s, 2/10 timeouts
- stage breakdown for a 2.8s request:
get_session=1041ms, compact=1717ms, redact=31ms, json_write=8ms
- compact stage iterates all messages for user_message_count and runs
recursive redact_session_data + json.dumps -- biggest single cost
Ref: perf/session-load-latency
Konstantin M
ce1aaf9d7b