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/library/mediainfo/ffprobe.go

@@ -95,6 +95,16 @@ func ExtractMediaInfo(ctx context.Context, ffprobePath, filePath string) (*Media
 	if integ := assessIntegrity(stderr.String(), mi); integ != nil {
 		mi.Integrity = integ
 	}
+	// Append external sidecar subtitles (a .srt/.ass next to the video, or a
+	// Subs/ bundle) AFTER the embedded streams, so embedded keep slice positions
+	// == their 0:s:N index. Local files only — a remote URL has no directory to
+	// scan (debrid streams rely on embedded subs from the URL). Best-effort:
+	// DiscoverSidecarSubtitles returns nil on an unreadable dir.
+	if !strings.Contains(filePath, "://") {
+		if ext := DiscoverSidecarSubtitles(filePath); len(ext) > 0 {
+			mi.Subtitles = append(mi.Subtitles, ext...)
+		}
+	}
 	return mi, nil
 }