feat(stream): burn bitmap (PGS/DVB) subtitles into the video via overlay

Bitmap subs can't be served as WebVTT, so the user picks one and the daemon
re-encodes with it overlaid. HLSSessionConfig.BurnSubtitleIndex (*int, nil=no
burn) flows into the cache key + a -filter_complex graph:
  [0✌️0]<vchain>[base];[0:s:N][base]scale2ref[sub][base2];[base2][sub]overlay[vout]
Overlay after the tonemap (SDR subs keep brightness); scale2ref fits the PGS
canvas to the output. Invalid/text/out-of-range index -> clean-encode fallback.
IsTextSubtitle now includes "text" (parity with the web classifier).

internal/engine/probe.go

@@ -61,7 +61,7 @@ type ProbeSubtitleTrack struct {
 // without re-rendering. Bitmap subs (PGS, DVB) need burn-in.
 func (s ProbeSubtitleTrack) IsTextSubtitle() bool {
 	switch s.Codec {
-	case "subrip", "srt", "ass", "ssa", "webvtt", "mov_text":
+	case "subrip", "srt", "ass", "ssa", "webvtt", "mov_text", "text":
 		return true
 	}
 	return false