feat(openai): Mobile RT 补全 plan_type、精确匹配账号、刷新时自动设置隐私

1. accounts/check 补全 plan_type：当 id_token 缺少 plan_type（如 Mobile RT），
   自动调用 accounts/check 端点获取订阅类型
2. orgID 精确匹配账号：从 JWT 提取 poid 匹配正确账号，避免 Go map
   遍历顺序随机导致 plan_type 不稳定
3. RT 刷新时设置隐私：调用 disableOpenAITraining 关闭训练数据共享，
   结果存入 extra.privacy_mode，后续跳过重复设置

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/internal/service/openai_privacy_service.go

@@ -69,6 +69,139 @@ func disableOpenAITraining(ctx context.Context, clientFactory PrivacyClientFacto
 	return PrivacyModeTrainingOff
 }
 
+// ChatGPTAccountInfo 从 chatgpt.com/backend-api/accounts/check 获取的账号信息
+type ChatGPTAccountInfo struct {
+	PlanType string
+	Email    string
+}
+
+const chatGPTAccountsCheckURL = "https://chatgpt.com/backend-api/accounts/check/v4-2023-04-27"
+
+// fetchChatGPTAccountInfo calls ChatGPT backend-api to get account info (plan_type, etc.).
+// Used as fallback when id_token doesn't contain these fields (e.g., Mobile RT).
+// orgID is used to match the correct account when multiple accounts exist (e.g., personal + team).
+// Returns nil on any failure (best-effort, non-blocking).
+func fetchChatGPTAccountInfo(ctx context.Context, clientFactory PrivacyClientFactory, accessToken, proxyURL, orgID string) *ChatGPTAccountInfo {
+	if accessToken == "" || clientFactory == nil {
+		return nil
+	}
+
+	ctx, cancel := context.WithTimeout(ctx, 15*time.Second)
+	defer cancel()
+
+	client, err := clientFactory(proxyURL)
+	if err != nil {
+		slog.Debug("chatgpt_account_check_client_error", "error", err.Error())
+		return nil
+	}
+
+	var result map[string]any
+	resp, err := client.R().
+		SetContext(ctx).
+		SetHeader("Authorization", "Bearer "+accessToken).
+		SetHeader("Origin", "https://chatgpt.com").
+		SetHeader("Referer", "https://chatgpt.com/").
+		SetHeader("Accept", "application/json").
+		SetSuccessResult(&result).
+		Get(chatGPTAccountsCheckURL)
+
+	if err != nil {
+		slog.Debug("chatgpt_account_check_request_error", "error", err.Error())
+		return nil
+	}
+
+	if !resp.IsSuccessState() {
+		slog.Debug("chatgpt_account_check_failed", "status", resp.StatusCode, "body", truncate(resp.String(), 200))
+		return nil
+	}
+
+	info := &ChatGPTAccountInfo{}
+
+	accounts, ok := result["accounts"].(map[string]any)
+	if !ok {
+		slog.Debug("chatgpt_account_check_no_accounts", "body", truncate(resp.String(), 300))
+		return nil
+	}
+
+	// 优先匹配 orgID 对应的账号（access_token JWT 中的 poid）
+	if orgID != "" {
+		if matched := extractPlanFromAccount(accounts, orgID); matched != "" {
+			info.PlanType = matched
+		}
+	}
+
+	// 未匹配到时，遍历所有账号：优先 is_default，次选非 free
+	if info.PlanType == "" {
+		var defaultPlan, paidPlan, anyPlan string
+		for _, acctRaw := range accounts {
+			acct, ok := acctRaw.(map[string]any)
+			if !ok {
+				continue
+			}
+			planType := extractPlanType(acct)
+			if planType == "" {
+				continue
+			}
+			if anyPlan == "" {
+				anyPlan = planType
+			}
+			if account, ok := acct["account"].(map[string]any); ok {
+				if isDefault, _ := account["is_default"].(bool); isDefault {
+					defaultPlan = planType
+				}
+			}
+			if !strings.EqualFold(planType, "free") && paidPlan == "" {
+				paidPlan = planType
+			}
+		}
+		// 优先级：default > 非 free > 任意
+		switch {
+		case defaultPlan != "":
+			info.PlanType = defaultPlan
+		case paidPlan != "":
+			info.PlanType = paidPlan
+		default:
+			info.PlanType = anyPlan
+		}
+	}
+
+	if info.PlanType == "" {
+		slog.Debug("chatgpt_account_check_no_plan_type", "body", truncate(resp.String(), 300))
+		return nil
+	}
+
+	slog.Info("chatgpt_account_check_success", "plan_type", info.PlanType, "org_id", orgID)
+	return info
+}
+
+// extractPlanFromAccount 从 accounts map 中按 key（account_id）精确匹配并提取 plan_type
+func extractPlanFromAccount(accounts map[string]any, accountKey string) string {
+	acctRaw, ok := accounts[accountKey]
+	if !ok {
+		return ""
+	}
+	acct, ok := acctRaw.(map[string]any)
+	if !ok {
+		return ""
+	}
+	return extractPlanType(acct)
+}
+
+// extractPlanType 从单个 account 对象中提取 plan_type
+func extractPlanType(acct map[string]any) string {
+	if account, ok := acct["account"].(map[string]any); ok {
+		if planType, ok := account["plan_type"].(string); ok && planType != "" {
+			return planType
+		}
+	}
+	if entitlement, ok := acct["entitlement"].(map[string]any); ok {
+		if subPlan, ok := entitlement["subscription_plan"].(string); ok && subPlan != "" {
+			return subPlan
+		}
+	}
+	return ""
+}
+
 func truncate(s string, n int) string {
 	if len(s) <= n {
 		return s