perf(webui): auto-log slow /api/session requests with stage breakdown

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

api/routes.py

@@ -11998,13 +11998,17 @@ def handle_get(handler, parsed) -> bool:
             _t5 = _time.monotonic()
             resp = j(handler, {"session": redact})
             _t6 = _time.monotonic()
-            if _debug_slow:
+            _total_ms = (_t6 - _t0) * 1000
+            # Always log when slow (>2s) so we don't need HERMES_DEBUG_SLOW env var
+            # to diagnose latency regressions. Opt-in env var still forces
+            # logging on every request for development.
+            if _debug_slow or _total_ms >= 2000:
                 logger.warning(
                     "[SLOW] session_id=%s get_session=%.1fms model_resolve=%.1fms "
                     "compact=%.1fms redact=%.1fms json_write=%.1fms total=%.1fms",
                     sid,
                     (_t2-_t1)*1000, (_t3-_t2)*1000, (_t4-_t3)*1000,
-                    (_t5-_t4)*1000, (_t6-_t5)*1000, (_t6-_t0)*1000,
+                    (_t5-_t4)*1000, (_t6-_t5)*1000, _total_ms,
                 )
             return resp
         except KeyError: