feat(audio): ffmpeg adapter -- always transcode to 16 kHz mono PCM (ADR-033)

Replaces whisper-server --convert. A zero exit that wrote nothing is an error.
Also fixes the red test's stub, which wrote an empty file and so (correctly)
tripped the no-output check.

Checked against real ffmpeg on iguana with the same argv: AAC .m4a in,
pcm_s16le 16000 Hz mono WAV out, duration preserved.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

internal/adapters/audio/ffmpeg.go

@@ -0,0 +1,45 @@
+package audio
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"os/exec"
+	"strings"
+)
+
+// FFmpeg converts any audio container to the strict format whisper wants: 16 kHz
+// mono signed 16-bit PCM WAV (ADR-033). It ALWAYS transcodes, even when the input
+// already is that format — a Teams export often is, and the transcode is then
+// nearly free; assuming it would be the bug. This replaces whisper-server's
+// --convert, whose --tmp-dir is created at process start and rots under macOS
+// periodic cleanup.
+type FFmpeg struct {
+	// Bin is the ffmpeg executable ("ffmpeg" resolves via PATH).
+	Bin string
+}
+
+// ToWAV writes src, converted, to dst. Video streams are dropped (-vn): by now the
+// file has already been probed as audio-only, but a cover-art stream must not end
+// up in the WAV.
+func (f *FFmpeg) ToWAV(ctx context.Context, src, dst string) error {
+	bin := f.Bin
+	if bin == "" {
+		bin = "ffmpeg"
+	}
+	cmd := exec.CommandContext(ctx, bin,
+		"-y", "-v", "error", "-i", src, "-vn", "-ac", "1", "-ar", "16000", "-c:a", "pcm_s16le", "-f", "wav", dst)
+	cmd.WaitDelay = killGrace
+	if out, err := cmd.CombinedOutput(); err != nil {
+		if ctxErr := ctx.Err(); ctxErr != nil {
+			return fmt.Errorf("audio: ffmpeg: %w", ctxErr)
+		}
+		return fmt.Errorf("audio: ffmpeg: %w: %s", err, strings.TrimSpace(string(out)))
+	}
+	// A zero exit that wrote nothing is still a failure: never hand whisper a
+	// missing file and let the error surface three layers later.
+	if info, err := os.Stat(dst); err != nil || info.Size() == 0 {
+		return fmt.Errorf("audio: ffmpeg wrote no output at %s", dst)
+	}
+	return nil
+}

internal/adapters/audio/ffmpeg_test.go

@@ -18,7 +18,9 @@ func TestFFmpegToWAV_AsksForSixteenKilohertzMonoPCMAlways(t *testing.T) {
 	// Always transcode, even when the input already is 16 kHz mono: a Teams export
 	// often is, and the point is to not ASSUME (ADR-033). Records the argv.
 	rec := filepath.Join(t.TempDir(), "argv")
-	bin := stub(t, `printf '%s\n' "$@" > `+rec+`; : > "$(eval echo \${$#})"`)
+	bin := stub(t, `printf '%s\n' "$@" > `+rec+`
+for a; do last="$a"; done
+printf data > "$last"`)
 
 	err := (&audio.FFmpeg{Bin: bin}).ToWAV(context.Background(), "/spool/in", filepath.Join(t.TempDir(), "out.wav"))