fix(backtest): benchmark全链路透传(API→runner→engine) + metrics分支用_dcfg修cfg=None导致relative_metrics空

sanguo_api/routes.py

@@ -75,7 +75,8 @@ async def submit_cta(req: CtaBacktestRequest):
         params=req.params,
         start=req.start,
         end=req.end,
-        cfg=None
+        cfg=None,
+        benchmark=req.benchmark
     )
     return {"task_id": tid}