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/config/config.go

@@ -189,6 +189,14 @@ type LibraryConfig struct {
 	AutoScan     bool   `toml:"auto_scan"`     // enable daily auto-scan in daemon (default true)
 	ScanInterval string `toml:"scan_interval"` // e.g. "24h", "12h", "6h" (default "24h")
 	AllowDelete  bool   `toml:"allow_delete"`  // allow web UI to request file deletion from disk
+
+	// Sidecar caching: extract text subtitles (WebVTT) and thumbnail frames once
+	// during the library scan and store them in a hidden ".unarr" dir next to the
+	// media file, so the stream handlers serve them instantly instead of running
+	// ffmpeg per request (and so huge remuxes don't hit the on-demand HTTP
+	// timeout). Both default true; disable to save the disk/CPU of pre-extraction.
+	CacheSubtitles  bool `toml:"cache_subtitles"`  // default true
+	CacheThumbnails bool `toml:"cache_thumbnails"` // default true
 }
 
 // Default returns a Config with sensible defaults. Used both for fresh
@@ -255,9 +263,11 @@ func Default() Config {
 			Locale:  "en",
 		},
 		Library: LibraryConfig{
-			AutoScan:     true,
-			ScanInterval: "24h",
-			Workers:      8,
+			AutoScan:        true,
+			ScanInterval:    "24h",
+			Workers:         8,
+			CacheSubtitles:  true,
+			CacheThumbnails: true,
 		},
 	}
 }
@@ -321,6 +331,16 @@ func applyDefaults(cfg *Config, meta toml.MetaData) {
 		cfg.General.Country = "US"
 	}
 
+	// Sidecar caching defaults ON for existing configs that predate these keys —
+	// it only adds small hidden files next to media and makes subs/thumbnails
+	// instant. Power users can set them false explicitly to opt out.
+	if !meta.IsDefined("library", "cache_subtitles") {
+		cfg.Library.CacheSubtitles = true
+	}
+	if !meta.IsDefined("library", "cache_thumbnails") {
+		cfg.Library.CacheThumbnails = true
+	}
+
 	if !meta.IsDefined("downloads", "transcode", "enabled") {
 		cfg.Download.Transcode.Enabled = true
 	}