Fix infinite retry loop in video upload transcode

Live logs (pulled via `hf spaces logs`) showed 39 repeated failures
alternating between "SDR_Render__h264.mp4" and "SDR_Render__gr_conv.mp4"
"No such file or directory" for a single upload — an infinite loop
between _transcode_for_browser and Gradio's own internal conversion,
each re-triggering on the other's (since-cleaned-up) output via the
cross-tab _sync propagation.

_transcode_for_browser now short-circuits to a no-op gr.update() when
its input is already one of these generated filenames, or no longer
exists on disk — breaking the cycle regardless of the exact re-trigger
mechanism, since there's no longer a path for a re-triggered handler to
chase.

app.py

@@ -338,7 +338,7 @@ def strip_audio_from_video(video_path: str, output_path: str) -> None:
             pix_fmt="yuv420p", an=None,
         ).run(overwrite_output=True, quiet=True)
 
-def _transcode_for_browser(video_path: str) -> str:
+def _transcode_for_browser(video_path: str):
     """Re-encode uploaded video to H.264/AAC MP4 so the browser preview widget can play it.
 
     Returns a NEW path in a fresh /tmp/gradio/ subdirectory. Gradio probes the
@@ -346,9 +346,26 @@ def _transcode_for_browser(video_path: str) -> str:
     slow fallback converter. The in-place overwrite approach loses the race
     because Gradio probes the original path at upload time before this callback runs.
     Only called on upload — not during generation.
+
+    Cross-tab sync (_sync) mirrors whatever this returns onto the other two
+    tabs' Video components, and each of THEIR .upload() handlers can re-fire
+    on that reassignment — without the guards below, that reprocesses an
+    already-transcoded "_h264.mp4"/"_gr_conv.mp4" file, and if it has since
+    been cleaned up this turns into an infinite failed-retry loop (observed
+    in prod: 39 repeated "No such file" failures alternating between the two
+    names for one upload). Both guards return gr.update() (no-op, doesn't
+    touch the component's value) instead of re-assigning a path, so there's
+    nothing for a re-triggered handler to chase.
     """
     if video_path is None:
         return video_path
+    base = os.path.basename(video_path)
+    if base.endswith("_h264.mp4") or base.endswith("_gr_conv.mp4"):
+        print(f"[transcode_for_browser] already processed ({base}), skipping")
+        return gr.update()
+    if not os.path.exists(video_path):
+        print(f"[transcode_for_browser] input vanished before processing: {video_path!r}")
+        return gr.update()
     try:
         probe     = ffmpeg.probe(video_path)
         has_audio = any(s["codec_type"] == "audio" for s in probe.get("streams", []))