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/lang.go

@@ -64,7 +64,13 @@ var langNormalize = map[string]string{
 	"mlt": "mt", "mt": "mt",
 	"swa": "sw", "sw": "sw",
 	"afr": "af", "af": "af",
-	"lat": "la", "la": "la",
+	"kan": "kn", "kn": "kn",
+	"mal": "ml", "ml": "ml",
+	"mar": "mr", "mr": "mr",
+	"pan": "pa", "pa": "pa",
+	"guj": "gu", "gu": "gu",
+	"kann": "kn",
+	"lat":  "la", "la": "la",
 
 	// Full English names (ffprobe sometimes returns these instead of codes)
 	"english": "en", "spanish": "es", "french": "fr", "german": "de",