Test background music side-chain ducking

tests/test_mix_audio.py

@@ -95,3 +95,36 @@ async def test_add_audio_rejects_invalid_bitrate(tmp_path: Path) -> None:
     sfx = _media(tmp_path / "one.wav", "one.wav", "audio/wav")
     with pytest.raises(InputError, match="supported AAC bitrate"):
         await add_audio(FakeFFmpeg(), [video, sfx], {"audio_bitrate": "999k"}, tmp_path / "out")
+
+
+@pytest.mark.asyncio
+async def test_duck_background_music_uses_voice_sidechain_and_preserves_video(tmp_path: Path) -> None:
+    from app.operations.mix_audio import duck_background_music
+
+    video = _media(
+        tmp_path / "voice.mp4", "voice.mp4", "video/mp4", duration=18.0,
+        metadata={"audio_streams": [{"codec": "aac"}]},
+    )
+    music = _media(tmp_path / "music.mp3", "music.mp3", "audio/mpeg")
+    ffmpeg = FakeFFmpeg()
+    result = await duck_background_music(
+        ffmpeg, [video, music], {"music_volume": 0.2, "ratio": 8, "attack": 20, "release": 300},
+        tmp_path / "out",
+    )
+    command = [str(value) for value in ffmpeg.args]
+    filt = command[command.index("-filter_complex") + 1]
+    assert "[music][voice]sidechaincompress=threshold=0.03:ratio=8:attack=20:release=300[ducked]" in filt
+    assert "[voice][ducked]amix=inputs=2:duration=first:normalize=0[mixed]" in filt
+    assert command[command.index("-c:v") + 1] == "copy"
+    assert result.metadata["mix_mode"] == "sidechain_ducking"
+
+
+@pytest.mark.asyncio
+async def test_duck_background_music_rejects_silent_video(tmp_path: Path) -> None:
+    from app.operations.mix_audio import duck_background_music
+
+    video = _media(tmp_path / "silent.mp4", "silent.mp4", "video/mp4", duration=8.0,
+                   metadata={"audio_streams": []})
+    music = _media(tmp_path / "music.mp3", "music.mp3", "audio/mpeg")
+    with pytest.raises(InputError, match="requires narration audio"):
+        await duck_background_music(FakeFFmpeg(), [video, music], {}, tmp_path / "out")