yt-dlp pulls metadata (title, description, uploader, thumbnail) and
bestaudio (opus). faster-whisper transcribes; audio file removed after.
Creates a refs row with kind='video' and source_kind='youtube' for
YouTube URLs, generic 'video' otherwise. Idempotent on
sha256(space_id + url) via refs.external_id.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
