feat(agent): store ffmpeg's actual failure reason in the job's error field

sample_frames_from_url now returns (frames, reason) — reason carries the
SPECIFIC cause on failure (ffmpeg's stderr tail, e.g. "moov atom not found",
or the timeout) instead of only logging it agent-side. The worker folds it
into the failure it reports, so curator's GpuJob.error reads e.g.

  no frames sampled from video — ffmpeg exit 183: moov atom not found ...

instead of the bare "(unprocessable)". The errored-jobs list becomes
self-describing: after a retry sweep, surviving errors name their real
defect without needing the agent log. Return-value plumbing (not shared
state) so concurrent downloaders stay isolated. Agent VERSION → 2026-07-02.2.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

agent/fc_agent/worker.py

@@ -632,7 +632,7 @@ class Worker:
             # mid-download was the failure loop). Environment-agnostic + resilient.
             url = f"{self.cfg.fc_url}{job['image_url']}"
             with self._timed("decode"):
-                frames = media.sample_frames_from_url(
+                frames, ffmpeg_err = media.sample_frames_from_url(
                     url, job.get("frame_interval_seconds", 4.0),
                     job.get("max_frames", 64),
                     headers=self._auth_header, timeout=self.cfg.ffmpeg_timeout,
@@ -644,11 +644,14 @@ class Worker:
                 if self._stopped(stop_evt):
                     raise requests.ConnectionError("stopped during video sampling")
                 # Else couldn't sample. If curator is up, the file is unprocessable
-                # → a job fault (fail it, don't re-lease forever). If curator is
-                # unreachable, it's transient → let the loop back off + retry
-                # (survives a redeploy). ConnectionError is caught as transient.
+                # → a job fault: fail it WITH ffmpeg's reason, so the job's stored
+                # error says e.g. "moov atom not found" instead of a bare
+                # "unprocessable". If curator is unreachable, it's transient → let
+                # the loop back off + retry (ConnectionError is caught as such).
                 if self.client.is_reachable():
-                    raise RuntimeError("no frames sampled from video (unprocessable)")
+                    raise RuntimeError(
+                        f"no frames sampled from video — {ffmpeg_err or 'unknown reason'}"
+                    )
                 raise requests.ConnectionError("curator unreachable during video sampling")
             # Temporal dedup: a near-static video re-runs the whole detect+embed
             # chain on ~identical frames — drop near-dups HERE (CPU) pre-GPU.