fix(agent): stop the downloader pool stampeding a slow curator (congestion collapse)

Operator hit an outage after the machine slept overnight: the agent showed
"curator unreachable" in a loop while curator's API (lease) was actually fine and
the browser could still load images — just slowly. Root cause is a feedback loop
in the new pipeline: every download streams a full original through curator's
single Python file-serving path, and the autoscaler grows DOWNLOADERS whenever the
buffer is empty. When downloads are merely SLOW/failing, the buffer is empty for
that reason — so the agent piled on more concurrent large-file GETs, saturating
curator's web workers + NFS, which slowed curator (and its browser) further and
produced more failures → more downloaders. Classic congestion collapse.

- Failure-aware autoscaling: if transient download failures rose since the last
  decision, SHRINK the downloader pool toward the floor instead of growing — the
  empty buffer is caused by failures, not the GPU starving. It ramps back up only
  once downloads succeed again.
- DL_MAX 24 → 8: 24 concurrent large-file downloads through one Python serving
  path is too many; 8 keeps a fast GPU fed without stampeding curator.
- fetch_image timeout 180 → (10, 60): the read timeout is between-bytes, so a
  large-but-flowing download still completes, but a stuck/dead connection fails in
  60s instead of hanging a downloader for 3 min and piling up stuck requests.

Build marker 2026-07-01.4.

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

agent/fc_agent/client.py

@@ -113,7 +113,11 @@ class FcClient:
 
     def fetch_image(self, image_url: str) -> bytes:
         # image_url is a server-relative path ("/images/...").
-        r = self.s.get(f"{self.base}{image_url}", timeout=180)
+        # timeout=(connect, read): the read timeout is BETWEEN-BYTES, not total,
+        # so a large-but-flowing download still completes — but a stuck/dead
+        # connection (curator overloaded) fails in 60s instead of hanging a
+        # downloader for 180s and piling up concurrent stuck requests on curator.
+        r = self.s.get(f"{self.base}{image_url}", timeout=(10, 60))
         r.raise_for_status()
         return r.content