fix: 渲染引擎flag解析增加调试日志 + 统一fallback行为

- 三个入口(compose_video/edit_plan/generate_video)都增加flag配置详情日志
  （enabled/percentage/whitelist/default_engine），灰度期间便于排查
- 统一fallback行为：异常时全部fallback到legacy（保守策略）
  之前generate_video异常时fallback到unified，与其他入口不一致

apps/worker/worker_app/tasks/compose_video.py

@@ -59,6 +59,7 @@ 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
@@ -76,10 +77,12 @@ def compose_video(self, job_id: str, **kwargs):
             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)