fix: preserve all audio tracks and align language metadata indices

Bug #5: extractAudio invoked ffmpeg without `-map`, so default stream
selection kept only one audio track from sources with multiple audio
streams (e.g. eng/fra/jpn Blu-rays). It now enumerates audio streams
from the streamLangs already fetched by the caller and runs one
`ffmpeg -map 0🅰️<n>` per stream, writing audio.<n>.wav.

Bug #11: the audio language metadata loop in encodeVideo computed the
output index by counting source-side audio streams with a lower Index,
which drifted when some source streams lacked a language tag. It now
walks opusFiles in output order and looks up the language at the
matching source-audio position via a sorted helper.

These ship together because #11 was masked by #5: when only one audio
track survived extraction, the broken index calculation never produced
a visible misalignment. Fixing #5 alone would have caused multi-track
outputs with shuffled language tags; both fixes are required to land
correct multi-track output.

internal/encoder/encoder.go

@@ -7,6 +7,7 @@ import (
 	"os/exec"
 	"path/filepath"
 	"regexp"
+	"sort"
 	"strconv"
 	"strings"
 
@@ -223,7 +224,7 @@ func (e *Encoder) calculateZscaleWidth(path string, originalHeight int) (string,
 
 func (e *Encoder) Transcode(input string, job *types.Job, metadata *types.Metadata, interlaced bool, streamLangs []StreamMetadata) error {
 	dir := filepath.Dir(input)
-	audioWavs, err := e.extractAudio(input, dir)
+	audioWavs, err := e.extractAudio(input, dir, streamLangs)
 	if err != nil {
 		return fmt.Errorf("extracting audio: %w", err)
 	}
@@ -242,16 +243,39 @@ func (e *Encoder) Transcode(input string, job *types.Job, metadata *types.Metada
 	return nil
 }
 
-func (e *Encoder) extractAudio(input, dir string) ([]string, error) {
-	cmd := exec.Command(e.ffmpegPath, "-i", input,
-		"-vn", "-c:a", "pcm_s16le", "-ar", "48000",
-		"-f", "wav", filepath.Join(dir, "audio.wav"))
-	if out, err := cmd.CombinedOutput(); err != nil {
-		return nil, fmt.Errorf("ffmpeg extract: %s %w", out, err)
+// audioStreamsInSourceOrder returns the audio entries of streamLangs sorted by
+// their source-side Index. The resulting slice position is the 0-based audio
+// stream index used by ffmpeg selectors like `0:a:<n>`.
+func audioStreamsInSourceOrder(streamLangs []StreamMetadata) []StreamMetadata {
+	var audio []StreamMetadata
+	for _, s := range streamLangs {
+		if s.CodecType == "audio" {
+			audio = append(audio, s)
+		}
+	}
+	sort.Slice(audio, func(i, j int) bool { return audio[i].Index < audio[j].Index })
+	return audio
+}
+
+func (e *Encoder) extractAudio(input, dir string, streamLangs []StreamMetadata) ([]string, error) {
+	audio := audioStreamsInSourceOrder(streamLangs)
+	if len(audio) == 0 {
+		return nil, fmt.Errorf("no audio streams found")
 	}
 
-	matches, _ := filepath.Glob(filepath.Join(dir, "audio.wav"))
-	return matches, nil
+	wavs := make([]string, 0, len(audio))
+	for srcAudioIndex := range audio {
+		wavPath := filepath.Join(dir, fmt.Sprintf("audio.%d.wav", srcAudioIndex))
+		cmd := exec.Command(e.ffmpegPath, "-i", input,
+			"-map", fmt.Sprintf("0:a:%d", srcAudioIndex),
+			"-vn", "-c:a", "pcm_s16le", "-ar", "48000",
+			"-f", "wav", wavPath)
+		if out, err := cmd.CombinedOutput(); err != nil {
+			return wavs, fmt.Errorf("ffmpeg extract a:%d: %s %w", srcAudioIndex, out, err)
+		}
+		wavs = append(wavs, wavPath)
+	}
+	return wavs, nil
 }
 
 func (e *Encoder) encodeOpus(wavs []string, dir string) ([]string, error) {
@@ -319,17 +343,24 @@ func (e *Encoder) encodeVideo(input string, opusFiles []string, job *types.Job,
 	args = append(args, "-c:s", "copy")
 	args = append(args, "-map_metadata", "-1")
 
-	for _, stream := range streamLangs {
-		if stream.CodecType == "audio" && stream.Language != "" {
-			idx := 1
-			for _, s := range streamLangs {
-				if s.CodecType == "audio" && s.Index < stream.Index {
-					idx++
-				}
-			}
-			args = append(args, fmt.Sprintf("-metadata:s:a:%d", idx-1), fmt.Sprintf("language=%s", stream.Language))
-			fmt.Printf("DEBUG setting audio language: stream %d -> language=%s\n", idx-1, stream.Language)
+	// Walk opusFiles by output audio index (0..N-1) and look up the source
+	// audio language at the same source-audio-index position. This keeps the
+	// `-metadata:s:a:i` index aligned with the output stream order, regardless
+	// of whether some source streams lacked a language tag.
+	audioStreams := audioStreamsInSourceOrder(streamLangs)
+	for i := range opusFiles {
+		if i >= len(audioStreams) {
+			break
 		}
+		lang := audioStreams[i].Language
+		if lang == "" {
+			continue
+		}
+		args = append(args, fmt.Sprintf("-metadata:s:a:%d", i), fmt.Sprintf("language=%s", lang))
+		fmt.Printf("DEBUG setting audio language: stream %d -> language=%s\n", i, lang)
+	}
+
+	for _, stream := range streamLangs {
 		if stream.CodecType == "subtitle" && stream.Language != "" {
 			idx := 0
 			for _, s := range streamLangs {