fix(agent): short videos failed as "unprocessable" — fps filter emits 0 frames

Root cause of the "no frames sampled from video (unprocessable)" flood
(operator-flagged 2026-07-02, whole 62k-70k image block + others): the
sampler used `-vf fps=1/4`, and ffmpeg's fps filter emits round(duration/4)
frames — which is ZERO for any clip shorter than ~2s. Short animation loops
(0.5s, 1.75s — verified against two originals from different artists) are
complete, valid h264 videos; ffmpeg decoded them fine, emitted no frames,
exited 0, and the agent failed the job as unprocessable. Long videos worked,
so only the short-clip class flooded.

Fix: sample with select ("first frame always, then one per interval of
timestamp") + -fps_mode vfr, and scale=out_range=full so limited-range
yuv420p sources don't trip the mjpeg encoder's full-range strictness
(secondary failure observed on a 4440x2760 clip). Verified locally against
both failing originals (frames extracted, PIL-clean) and a synthetic 15s
video (4 frames at t=0/4/8/12 — long-video behavior unchanged).

Observability (why this hid for weeks): ffmpeg's stderr was discarded, so
every failure logged only "no frames sampled". stderr now goes to a temp
file and its tail is logged on any produced-no-frames/timeout failure — the
log names the actual ffmpeg reason from now on. Also: frames written before
a mid-stream ffmpeg error are now kept (partial > nothing).

VERSION → 2026-07-02.1.

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

agent/fc_agent/app.py

@@ -21,7 +21,7 @@ log = logging.getLogger("fc_agent.app")
 # Bump on every agent change. The page embeds this and /status reports it; the UI
 # warns to reload when they differ — so a stale browser-cached page can't be
 # mistaken for "the new image didn't deploy". (Belt-and-braces with no-store.)
-VERSION = "2026-07-01.10 · fix Status freeze (conchint.textContent destroyed #capn)"
+VERSION = "2026-07-02.1 · short videos sample again (select, not fps) + ffmpeg errors logged"
 
 logbuf.install()
 cfg = Config.from_env()