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_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"))