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/antigravity_gateway_service.go

@@ -441,13 +441,13 @@ func (s *AntigravityGatewayService) Forward(ctx context.Context, c *gin.Context,
 								_ = resp.Body.Close()
 								resp = retryResp
 								respBody = nil
-							}
-
+							} else {
 								// Retry still errored: replace error context with retry response.
 								retryBody, _ := io.ReadAll(io.LimitReader(retryResp.Body, 2<<20))
 								_ = retryResp.Body.Close()
 								respBody = retryBody
 								resp = retryResp
+							}
 						} else {
 							log.Printf("Antigravity account %d: signature retry request failed: %v", account.ID, retryErr)
 						}

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)
 	}