Detect and fix non-4:2:0 chroma in H.264 MP4s during transcode

Browsers only decode H.264 4:2:0 (Baseline/Main/High). Files reported
as codec_name=h264 could still be High 4:2:2/4:4:4 profile if an
earlier transcode pass (or the source) used non-standard chroma —
ffprobe/ffmpeg read these fine, but real browsers reject them as
unsupported. update.rb now also checks pix_fmt and re-transcodes if
it isn't yuv420p/yuvj420p, and forces -pix_fmt yuv420p on output so
this can't recur.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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