fix(agent): stream videos via ffmpeg-from-URL instead of downloading the whole file

The failing "poison" jobs were 800MB+ 4K VR videos: the agent pulled the ENTIRE
file into memory (r.content) just to sample a few frames, which buffered ~1GB in
RAM and — on any slow/contended media store — got cut off mid-download
(ChunkedEncodingError), failed, and re-leased forever. Measured the media read at
~4–6 MB/s (raw off the share, curator out of the path), so no serving-layer tweak
helps; the file simply shouldn't be fully downloaded.

Environment-agnostic fix (works for any deployment, completes even when slow):
- media.sample_frames_from_url(): point ffmpeg straight at curator's /images URL.
  It Range-reads only the video index + up to max_frames of content — never the
  whole file — and reconnect flags resume a dropped transfer instead of failing.
  Generous, env-tunable timeout (FFMPEG_TIMEOUT, default 1200s) = completion over
  speed. Removes the bytes-based sample_frames (dead once videos stream).
- worker._download_decode: videos now stream (no fetch_image, no RAM blowup);
  stills still download+decode. On an ffmpeg miss, probe curator liveness
  (client.is_reachable) → fail the job if curator is up (unprocessable file, stops
  the infinite re-lease) vs release if curator is down (transient, survives a
  redeploy). Auth header passed so it works whether or not /images is gated.

Build marker 2026-07-01.6. Refs issue #1225.

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

agent/fc_agent/client.py

@@ -121,6 +121,16 @@ class FcClient:
         r.raise_for_status()
         return r.content
 
+    def is_reachable(self) -> bool:
+        """Cheap 'is curator responding at all right now?' check. Used to decide,
+        when a video can't be sampled, between a transient outage (keep retrying —
+        survives a redeploy) and an unprocessable file (fail it, don't loop)."""
+        try:
+            r = self.s.get(f"{self.base}/api/gpu/status", timeout=5)
+            return r.status_code < 500
+        except requests.RequestException:
+            return False
+
     def queue_status(self) -> dict:
         # Short timeout: this backs the UI /status poll, so a busy curator must
         # not hang the page for long (the GPU meters poll /gpu separately).