Fix ffmpeg stream-copy crash in strip_audio_from_video

Falls back to re-encoding (libx264) when the source codec can't be
stream-copied into the mp4 container (e.g. VP9/webm, ProRes/mov),
instead of letting ffmpeg.Error crash the request.

app.py

@@ -293,10 +293,23 @@ def get_video_duration(video_path: str) -> float:
     return float(probe["format"]["duration"])
 
 def strip_audio_from_video(video_path: str, output_path: str) -> None:
-    """Write a silent copy of *video_path* to *output_path* (stream-copy, no re-encode)."""
-    ffmpeg.input(video_path).output(output_path, vcodec="copy", an=None).run(
-        overwrite_output=True, quiet=True
-    )
+    """Write a silent copy of *video_path* to *output_path*.
+
+    Tries a fast stream-copy first. Falls back to re-encoding when the
+    source codec can't be stream-copied into an mp4 container (e.g.
+    VP9/webm, ProRes/mov, or any upload that skipped/failed the
+    browser-preview H.264 transcode in _transcode_for_browser)."""
+    try:
+        ffmpeg.input(video_path).output(output_path, vcodec="copy", an=None).run(
+            overwrite_output=True, quiet=True
+        )
+    except ffmpeg.Error as e:
+        stderr = e.stderr.decode(errors="replace") if e.stderr else str(e)
+        print(f"[strip_audio_from_video] stream-copy failed, re-encoding instead: {stderr[-2000:]}")
+        ffmpeg.input(video_path).output(
+            output_path, vcodec="libx264", preset="fast", crf=18,
+            pix_fmt="yuv420p", an=None,
+        ).run(overwrite_output=True, quiet=True)
 
 def _transcode_for_browser(video_path: str) -> str:
     """Re-encode uploaded video to H.264/AAC MP4 so the browser preview widget can play it.
@@ -340,7 +353,8 @@ def _transcode_for_browser(video_path: str) -> str:
         print(f"[transcode_for_browser] transcoded to H.264: {out_path}")
         return out_path
     except Exception as e:
-        print(f"[transcode_for_browser] failed, using original: {e}")
+        stderr = e.stderr.decode(errors="replace") if isinstance(e, ffmpeg.Error) and e.stderr else str(e)
+        print(f"[transcode_for_browser] failed, using original: {stderr[-2000:]}")
         return video_path