fix: 优雅关停超时不再跳过 Cleanup，避免缓冲的用量/计费记录丢失

backend/cmd/server/main.go

@@ -183,7 +183,7 @@ func runMainServer() {
 	defer cancel()
 
 	if err := app.Server.Shutdown(ctx); err != nil {
-		log.Fatalf("Server forced to shutdown: %v", err)
+		log.Printf("Server forced to shutdown: %v", err)
 	}
 
 	log.Println("Server exited")