fix(lint): 修复 golangci-lint 检查问题

- usage_service: 修复 tx.Rollback 未检查错误返回值 (errcheck)
- antigravity_gateway: 修复重试逻辑中的无效赋值 (ineffassign)
- antigravity_gateway: 完善重试成功/失败的分支逻辑

backend/internal/service/usage_service.go

@@ -78,7 +78,7 @@ func (s *UsageService) Create(ctx context.Context, req CreateUsageLogRequest) (*
 
 	txCtx := ctx
 	if err == nil {
-		defer tx.Rollback()
+		defer func() { _ = tx.Rollback() }()
 		txCtx = dbent.NewTxContext(ctx, tx)
 	}