feat(unified-render): Phase 3 - 音频统一混音 + 灰度观测埋点

- 音频后处理：主图层concat拼接 + 独立音频轨amix混音
- 音视频分离：视频先渲染无声版，音频后处理后合并
- 支持音量调节（audio clip config.volume）
- 灰度埋点：渲染开始/视频渲染完成/音频混音完成/总完成 四个阶段日志
- 新增11个音频混音单元测试
- _resolve_layer_role 支持 audio role 映射

apps/worker/video_processing/render_adapter.py

@@ -140,7 +140,7 @@ class RenderAdapter:
                 )
 
             logger.info(
-                "开始渲染: plan_id=%s job_id=%s ready_clips=%d",
+                "开始渲染: plan_id=%s job_id=%s ready_clips=%d engine=unified",
                 plan_id,
                 job_id,
                 len(ready_clips),
@@ -176,6 +176,18 @@ class RenderAdapter:
 
             self._report_progress(progress_cb, 100.0, "渲染完成")
 
+            logger.info(
+                "[render-adapter] render success: plan_id=%s job_id=%s engine=unified "
+                "duration=%.2fs file_size=%d resolution=%dx%d clip_count=%d",
+                plan_id,
+                job_id,
+                result.duration,
+                result.file_size,
+                result.width,
+                result.height,
+                len(ready_clips),
+            )
+
             return RenderAdapterResult(
                 success=True,
                 output_url=output_url or "",
@@ -188,7 +200,12 @@ class RenderAdapter:
             )
 
         except Exception as exc:
-            logger.exception("渲染失败: plan_id=%s", plan_id)
+            logger.exception(
+                "[render-adapter] render failed: plan_id=%s job_id=%s engine=unified error=%s",
+                plan_id,
+                job_id,
+                str(exc)[:200],
+            )
             return RenderAdapterResult(
                 success=False,
                 error_message=str(exc)[:500],