fix: 移除render_metrics依赖（该模块在develop上不存在），保持PR独立可合并

PR #248 核心是flag调试日志 + fallback统一，埋点应由专门的灰度观测指标PR负责。
移除compose_video中对render_metrics的引用，确保PR可独立合并、不引入运行时错误。

apps/worker/worker_app/tasks/compose_video.py

@@ -59,7 +59,6 @@ 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
@@ -77,12 +76,10 @@ def compose_video(self, job_id: str, **kwargs):
             config.get("default_engine"),
         )
 
-        # 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)
+        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)