On-demand WebVTT extraction re-ran ffmpeg on every /sub request and, for
50GB+ remuxes, couldn't finish a full text track within the 60s HTTP timeout
→ the web player got a 500 and no subtitles.
Extract each text subtitle ONCE — during the library scan (no HTTP deadline,
generous per-file timeout) and write-through on the first on-demand request —
into a hidden ".unarr/<name>.s<index>.vtt" sidecar next to the media file.
The /sub handler serves a fresh sidecar instantly (mtime-invalidated when the
media is replaced), so playback subtitles are instant and huge files work.
- mediainfo.sidecar: cache paths, mtime freshness, atomic write, ExtractSubtitleVTT,
IsTextSubtitleCodec (shared classifier, mirrors engine + web whitelists).
- library.PrewarmSidecars: bounded, idempotent, ctx-cancellable background pass
run after every scan (manual + daemon auto-scan).
- subtitleHandler: cache-read → hit; miss → extract → write-through.
- config: library.cache_subtitles (default true), wired via SetCacheSubtitles.
Local-only by design: nothing extracted is uploaded — the sidecar is the user's
own content, private to their disk.
