fix: AiGateway 缺少 @PostConstruct import 导致编译失败

cfc-backend/src/main/java/com/etotem/cfc/service/AiGateway.java

@@ -16,6 +16,7 @@ import org.springframework.web.client.RestTemplate;
 import java.util.*;
 import java.util.concurrent.atomic.AtomicInteger;
 import java.util.concurrent.atomic.AtomicLong;
+import javax.annotation.PostConstruct;
 
 /**
  * LangGraph Python 服务网关