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/client.py

@@ -4,6 +4,7 @@ The agent's ONLY contact with FC — lease/submit/heartbeat/fail + fetch image
 bytes, all over HTTP with the bearer token. No DB/Redis.
 """
 import requests
+from requests.adapters import HTTPAdapter
 
 
 class FcClient:
@@ -12,6 +13,11 @@ class FcClient:
         self.agent_id = agent_id
         self.s = requests.Session()
         self.s.headers["Authorization"] = f"Bearer {token}"
+        # Many worker threads share this Session; the default pool (10) would
+        # throttle them + spam "connection pool is full". Size it for the cap.
+        adapter = HTTPAdapter(pool_connections=64, pool_maxsize=64)
+        self.s.mount("http://", adapter)
+        self.s.mount("https://", adapter)
 
     def lease(self, batch_size: int) -> list[dict]:
         r = self.s.post(