fix(openai): 修复 usage 为空导致 panic（P0-02）

backend/internal/service/openai_gateway_service.go

@@ -969,6 +969,10 @@ func (s *OpenAIGatewayService) Forward(ctx context.Context, c *gin.Context, acco
 		}
 	}
 
+	if usage == nil {
+		usage = &OpenAIUsage{}
+	}
+
 	reasoningEffort := extractOpenAIReasoningEffort(reqBody, originalModel)
 
 	return &OpenAIForwardResult{