Direct googlevideo URLs are bound to the extractor's IP and expire, so the
browser fetching them from a different IP got intermittent 403s on playback.
Route playback through a same-origin /api/play proxy that fetches the stream
server-side (matching the extractor IP) with yt-dlp's own http_headers and
forwards Range headers for seeking; fall back to piping yt-dlp for SABR/itag-18
formats a plain GET can't fetch. Prefer adaptive streams over progressive in
the playback fallback order.
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