Previously a corrupt source (e.g. truncated mp4 with no moov atom) would be
re-queued on every directory scan: cleanup_partial_hls wipes the temp
playlist on ffmpeg failure, leaving no .m3u8 to short-circuit the next pass.
Mirrors the thumbnail .unsupported sentinel pattern: on ffmpeg failure,
write <playlist>.m3u8.unsupported, and treat its presence as "done" in both
the ScanDirectoryMessage filter and the QueueVideosMessage check. Delete
the sentinel to force a retry.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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