fix: address AI code review - probe defaults to no audio, safe af filter

- render_audio.py: simple mode builds af_parts list then joins into single -af (defensive against future multi-clip)
- unified_render_service.py: probe_has_audio exception now defaults to False with warning log (was unsafe True)
- test: add test_pass_through_probe_exception_defaults_to_no_audio

apps/worker/video_processing/render_audio.py

@@ -294,9 +294,13 @@ def concat_main_audio(
                 "-ac",
                 "2",
             ]
+            # 单 clip 简单模式：将所有音频滤镜合并为单个 -af 参数（避免多个 -af 冲突）
+            af_parts: list[str] = []
             vol = _clip_volume(clip)
             if abs(vol - 1.0) >= 1e-6:
-                command.extend(["-af", f"volume={vol:.4f}"])
+                af_parts.append(f"volume={vol:.4f}")
+            if af_parts:
+                command.extend(["-af", ",".join(af_parts)])
             if trim_start > 0:
                 command.extend(["-ss", f"{trim_start:.3f}"])
             if final_duration > 0:

apps/worker/video_processing/unified_render_service.py

@@ -1282,8 +1282,9 @@ class UnifiedRenderService:
         if role != "background" and clip_volume > 0:
             try:
                 has_audio = probe_has_audio(clip.local_path)
-            except Exception:
-                has_audio = True
+            except Exception as e:
+                logger.warning("[unified-render] 探测音频流失败，假设无音频: %s: %s", clip.local_path, e)
+                has_audio = False
         else:
             has_audio = False
         if has_audio:

tests/unit/test_original_audio_retained.py

@@ -191,3 +191,24 @@ class TestPassThroughAudioProbe:
     def test_clip_volume_static_helper(self):
         assert UnifiedRenderService._clip_volume(_clip("c1")) == 1.0
         assert UnifiedRenderService._clip_volume(_clip("c2", config={"volume": 0.7})) == pytest.approx(0.7)
+
+    def test_pass_through_probe_exception_defaults_to_no_audio(self):
+        """probe_has_audio 抛异常时必须默认无音频，不能盲目映射不存在的音轨。"""
+        clips = [_clip("c1")]
+        svc = _service(clips)
+        with (
+            patch("pathlib.Path.exists", return_value=True),
+            patch("video_processing.unified_render_service.probe_duration", return_value=5.0),
+            patch(
+                "video_processing.unified_render_service.probe_has_audio",
+                side_effect=RuntimeError("probe failed"),
+            ) as mock_probe,
+            patch("video_processing.unified_render_service.run_ffmpeg") as mock_run,
+        ):
+            layers = svc._group_clips_into_layers(clips)
+            has_audio = svc._render_pass_through(layers, Path("/tmp/out.mp4"), video_duration=5.0)
+
+        assert has_audio is False
+        mock_probe.assert_called()
+        cmd_str = " ".join(mock_run.call_args[0][0])
+        assert "aac" not in cmd_str