Never send a video as its own gallery card

When a video's first frame could not be read (a broken file, no decoder)
the card's thumbnail route served the whole video as the picture. It now
gets the same SVG placeholder with its size as a picture with no card.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

api/gallery_routes.py

@@ -159,9 +159,9 @@ async def api_output_thumbnail(request):
         if served is None:
             return picture_placeholder(file_path)
     elif ext in CARD_POSTER_VIDEO_EXTENSIONS:
-        poster = await asyncio.to_thread(_build_video_poster, file_path)
-        if poster:
-            served = poster
+        served = await asyncio.to_thread(_build_video_poster, file_path)
+        if not served:  # no first frame (a broken file, no decoder): never the video itself as a picture
+            return picture_placeholder(file_path)
     # Revalidate (cheap 304s): an output file can be replaced under the same name.
     return web.FileResponse(served, headers={'Cache-Control': 'no-cache'})