feat(subs): resilient subtitle extraction — sidecars, charset, torrent/debrid

Close the recurring "video has subtitles but the web player shows none" gap
with a source-agnostic pipeline:

- Discover EXTERNAL sidecar subs in the scan (Video.es.ass siblings + a Subs/
  bundle), parse lang/forced/SDH from the filename, skip VobSub (.sub+.idx).
  ffprobe-only scanning ignored these (ToonsHub/anime "MSubs" releases).
- Transcode sidecar charset -> UTF-8 before WebVTT (BOM/UTF-16/code-page by
  language). Chinese SCRIPT matters: chs/sc -> GBK, cht/tc/big5 -> Big5
  (decoding one as the other is mojibake).
- /sub now serves a standalone sidecar file (i=-1, p=file, &l=lang hint) and a
  remote debrid URL (ffmpeg reads http, no local stat) — not just embedded
  streams of a local file.
- probe.json emits a tokened vttUrl per TEXT track so torrent/debrid HLS streams
  (never library-scanned) get subtitles too. Embedded index is counted among
  embedded streams only, so -map 0:s:N stays aligned when sidecars are appended.

Tested against a real 347-file gallery: 26/26 sidecars and embedded ass/srt/
mov_text all extract to valid WebVTT; bitmap (pgs/dvd_subtitle) correctly stays
burn-in. Manual harness gated behind GALLERY_DIR.

internal/engine/hls.go

@@ -574,13 +574,18 @@ func (s *HLSSession) ProbeInfo() map[string]any {
 	}
 	subs := make([]map[string]any, 0, len(s.probe.SubtitleTracks))
 	for _, sb := range s.probe.SubtitleTracks {
+		// `external`/`path` let the stream server attach a tokened /sub vttUrl
+		// (path-addressed for sidecars, index-addressed for embedded). `path` is
+		// stripped after the URL is built so the raw path isn't doubled in JSON.
 		subs = append(subs, map[string]any{
-			"index":  sb.Index,
-			"lang":   sb.Lang,
-			"codec":  sb.Codec,
-			"title":  sb.Title,
-			"forced": sb.Forced,
-			"text":   sb.IsTextSubtitle(),
+			"index":    sb.Index,
+			"lang":     sb.Lang,
+			"codec":    sb.Codec,
+			"title":    sb.Title,
+			"forced":   sb.Forced,
+			"text":     sb.IsTextSubtitle(),
+			"external": sb.External,
+			"path":     sb.Path,
 		})
 	}
 	return map[string]any{