radio-t / stream-recorder / 25623157659

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Add post-recording VBR header fix to recorded MP3s

Recorded MP3s have variable bitrate (icecast source mixes 32-128 kbps
frames depending on speech vs silence) but no Xing/Info VBR header.
Without that header, players estimate duration from the first frame's
bitrate, which is wildly wrong: a 3h15m recording shows up as 8h42m in
Apple/Safari, and seeking past the real audio end stalls the player.

Fixed by remuxing the file with `ffmpeg -c copy -f mp3` after metadata
injection. ffmpeg adds a proper Xing header (frame-counted duration),
preserves all ID3 tags including TLEN and CHAP/CTOC frames, and doesn't
re-encode (~1s for a 140 MB file).

The post-processor no-ops cleanly when ffmpeg isn't on PATH, so dev/test
runs without the dependency still work.

11 of 23 new or added lines in 2 files covered. (47.83%)

957 of 1278 relevant lines covered (74.88%)

7.33 hits per line