feat(stream): cache extracted subtitles to a hidden .unarr sidecar

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.

internal/cmd/scan.go

@@ -16,6 +16,7 @@ import (
 	"github.com/torrentclaw/unarr/internal/agent"
 	"github.com/torrentclaw/unarr/internal/config"
 	"github.com/torrentclaw/unarr/internal/library"
+	"github.com/torrentclaw/unarr/internal/library/mediainfo"
 )
 
 func newScanCmd() *cobra.Command {
@@ -139,6 +140,20 @@ func runScan(dirPath string, workers int, ffprobePath string, noSync bool) error
 		return enc.Encode(cache)
 	}
 
+	// Pre-extract subtitle sidecars (text subs → WebVTT in a hidden ".unarr" dir)
+	// so playback gets instant subtitles and huge remuxes never hit the on-demand
+	// timeout. Best-effort + Ctrl-C interruptible (the scan itself is already saved).
+	if cfg.Library.CacheSubtitles {
+		if ff, err := mediainfo.ResolveFFmpeg(cfg.Library.FFmpegPath); err == nil {
+			fmt.Fprintf(os.Stderr, "  Pre-extracting subtitles to cache… (Ctrl-C to skip)\n")
+			library.PrewarmSidecars(ctx, cache, library.PrewarmOptions{
+				FFmpegPath:     ff,
+				CacheSubtitles: true,
+				Workers:        2,
+			})
+		}
+	}
+
 	// Sync to server
 	if !noSync {
 		return syncToServer(ctx, cfg, cache)