The node's auto-downloader fetched johnvansickle/gyan static builds, which
have no NVENC, so nvenc_* formats could never work on a machine without a
system NVENC ffmpeg.
- Switch the download source to BtbN static builds (compiled with
NVENC/CUDA; ~125 MB, ship all CPU codecs too).
- _get_ffmpeg(required_encoder) now self-provisions: when no existing
ffmpeg has the requested hardware encoder, download BtbN and prefer it;
download runs at most once per process.
- Add .gitignore for the runtime-provisioned ffmpeg_bin/ and pycache.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
