feat(agent): raise worker cap to 32 + size the HTTP pool for it (#114)

At 8 workers the GPU sat at ~5% util / <5GB VRAM — the pipeline is I/O-bound
(downloading + decoding images over HTTP), so the GPU starves until many workers
overlap that I/O. Raise MAX_CONCURRENCY 8→32 and make the UI worker control a
number input (reaching 32 by ±1 was tedious); the cap is reported via /status so
the UI clamps to it. Also size the shared requests pool (pool_maxsize=64) — the
default 10 would have throttled 32 workers + spammed "connection pool is full".

Verified by running; watch GPU util/VRAM climb as you dial up.

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

agent/fc_agent/worker.py

@@ -17,7 +17,10 @@ from .client import FcClient
 from .config import Config
 from .crops import crop_region
 
-MAX_CONCURRENCY = 8
+# Generous cap: the pipeline is usually I/O-bound (downloading + decoding images
+# over HTTP), so the GPU stays underused until many workers overlap that I/O.
+# Push it up while watching the GPU util + VRAM in the UI.
+MAX_CONCURRENCY = 32
 
 
 class _Slot:
@@ -74,6 +77,7 @@ class Worker:
             return {
                 "state": "running" if self._running else "stopped",
                 "concurrency": self._target,
+                "max_concurrency": MAX_CONCURRENCY,
                 "workers": len(self._slots),
                 "active": self._active,
                 "processed": self.processed,