fix(Antigravity): 支持无 project_id 的账户类型

- 移除 project_id 强制检查，部分账户类型 API 不返回此字段
- 重构：提取 antigravity.NewAPIRequest() 统一创建 API 请求
- quota_refresher: 无 project_id 时仍可更新 tier 信息

backend/internal/pkg/antigravity/client.go

@@ -1,6 +1,7 @@
 package antigravity
 
 import (
+	"bytes"
 	"context"
 	"encoding/json"
 	"fmt"
@@ -11,6 +12,19 @@ import (
 	"time"
 )
 
+// NewAPIRequest 创建 Antigravity API 请求（v1internal 端点）
+func NewAPIRequest(ctx context.Context, action, accessToken string, body []byte) (*http.Request, error) {
+	apiURL := fmt.Sprintf("%s/v1internal:%s", BaseURL, action)
+	req, err := http.NewRequestWithContext(ctx, http.MethodPost, apiURL, bytes.NewReader(body))
+	if err != nil {
+		return nil, err
+	}
+	req.Header.Set("Content-Type", "application/json")
+	req.Header.Set("Authorization", "Bearer "+accessToken)
+	req.Header.Set("User-Agent", UserAgent)
+	return req, nil
+}
+
 // TokenResponse Google OAuth token 响应
 type TokenResponse struct {
 	AccessToken  string `json:"access_token"`