fix(agent): unfreeze status view + smoothed throughput-aware autoscaler + log pane

Operator: the status tiles (state/active/processed) and the Start/Stop buttons
freeze while the GPU meters stay live. Root cause: /status made an INLINE
blocking curator call (queue_status) on every poll, and with curator buried
under a 112k-job backlog that call stalled — freezing the whole status refresh
(the GPU bars survived because /gpu is a lock-free local read). Made worse by the
old util-band autoscaler, which grew workers toward the 32 cap forever because
util plateaus ~50% on this IO-bound load and never hit the 70 grow threshold —
piling load onto curator and the agent process.

- /status is now a pure in-memory read: worker.status() is lock-free, and the
  curator queue snapshot is refreshed by a background poller (never inline).
- Autoscaler replaced with a smoothed, throughput-aware climb that SETTLES:
  samples util every 2s and EWMA-smooths it (raw util swings 0↔99), then every
  ~24s grows by one only while each grow keeps lifting smoothed jobs/s; when a
  grow stops helping it backs off one and holds, re-probing occasionally. No
  runaway, no flopping.
- GPU util bar now shows a smoothed value: the agent's own EWMA (util_smooth,
  exposed on /gpu) when running, else smoothed client-side — so it glides
  instead of bouncing 0↔99.
- act() aborts a slow Start/Stop POST after 8s so the buttons can't stick; the
  now-always-fast /status refresh recovers state regardless.
- Log pane: bound the page to the viewport (height:100vh) so the Logs card
  scrolls INTERNALLY instead of overflowing off-screen; cap the ring buffer at
  400 lines.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Ttrj5P7upUTueSfoJcxEqa

agent/fc_agent/logbuf.py

@@ -5,7 +5,7 @@ logging.Handler appends to it; the UI polls /logs."""
 import logging
 from collections import deque
 
-LINES: deque[str] = deque(maxlen=600)
+LINES: deque[str] = deque(maxlen=400)
 
 
 class RingHandler(logging.Handler):