feat: 接入渲染灰度观测指标到compose_video和stream_copy

- compose_video 入口接入 render_task_metrics 上下文
  自动记录：引擎选择、任务耗时、成功/失败、错误类型、活跃任务数
- unified_render_service 接入 stream_copy 埋点
  三种状态：hit（命中直通）/ miss（条件不满足跳过）/ fallback（失败回退）
  miss 原因归一化为8类：编码/分辨率/帧率/字幕/像素格式/裁剪/多片段/其他

apps/worker/video_processing/unified_render_service.py

@@ -728,6 +728,10 @@ class UnifiedRenderService:
                 self.plan.id,
                 reason,
             )
+            # 灰度观测：stream_copy 未命中
+            from video_processing.render_metrics import classify_stream_copy_miss_reason, record_stream_copy
+            miss_reason = classify_stream_copy_miss_reason(reason)
+            record_stream_copy("miss", miss_reason)
             return False
 
         # 构建 copy 命令
@@ -784,9 +788,15 @@ class UnifiedRenderService:
                     self.plan.id,
                     output_path.stat().st_size,
                 )
+                # 灰度观测：stream_copy 命中成功
+                from video_processing.render_metrics import record_stream_copy
+                record_stream_copy("hit", "成功")
                 return True
             else:
                 logger.warning("[unified-render] stream_copy 输出为空: plan_id=%s", self.plan.id)
+                # 灰度观测：stream_copy 失败回退（输出为空）
+                from video_processing.render_metrics import record_stream_copy
+                record_stream_copy("fallback", "输出为空")
                 return False
         except (subprocess.CalledProcessError, subprocess.TimeoutExpired) as e:
             logger.warning(
@@ -794,6 +804,9 @@ class UnifiedRenderService:
                 self.plan.id,
                 str(e)[:200],
             )
+            # 灰度观测：stream_copy 失败回退（ffmpeg错误）
+            from video_processing.render_metrics import record_stream_copy
+            record_stream_copy("fallback", "ffmpeg失败")
             # 清理可能的损坏输出文件
             if output_path.exists():
                 try:

apps/worker/worker_app/tasks/compose_video.py

@@ -63,15 +63,30 @@ def compose_video(self, job_id: str, **kwargs):
         # 判断使用哪个渲染引擎
         # 优先级：Redis Feature Flag（白名单 > 百分比） > 环境变量默认
         from video_processing.render_engine_resolver import get_render_engine_resolver
+        from video_processing.render_metrics import render_task_metrics
 
         resolver = get_render_engine_resolver()
         user_id = job.created_by_user_id or None
         engine = resolver.get_engine(user_id=user_id)
+        # 灰度期间打印详细 flag 配置，便于排查
+        config = resolver.get_config_snapshot()
+        logger.info(
+            "compose_video 引擎选择: job_id=%s engine=%s user_id=%s enabled=%s percentage=%s whitelist=%d default=%s",
+            job_id,
+            engine,
+            user_id,
+            config.get("enabled"),
+            config.get("percentage"),
+            len(config.get("whitelist", [])),
+            config.get("default_engine"),
+        )
 
-        if engine == "unified":
-            return _compose_with_unified_engine(self, job_service, job, plan_id, db)
-        else:
-            return _compose_with_legacy_engine(self, job_service, job, plan_id, db)
+        # 灰度观测指标埋点
+        with render_task_metrics(engine=engine, task_type="compose_video"):
+            if engine == "unified":
+                return _compose_with_unified_engine(self, job_service, job, plan_id, db)
+            else:
+                return _compose_with_legacy_engine(self, job_service, job, plan_id, db)
 
     except self.retry_exc as exc:
         logger.warning("视频合成重试中: job_id=%s, exc=%s", job_id, exc)