feat: 从 OpenAI JWT 提取 chatgpt_plan_type 并在前端展示

OAuth 授权和 token 刷新时从 id_token 的 OpenAI auth claim 中
提取 chatgpt_plan_type（plus/team/pro/free），存入 credentials，
账号管理页面 PlatformTypeBadge 显示订阅类型。

backend/internal/pkg/openai/oauth.go

@@ -268,6 +268,7 @@ type IDTokenClaims struct {
 type OpenAIAuthClaims struct {
 	ChatGPTAccountID string              `json:"chatgpt_account_id"`
 	ChatGPTUserID    string              `json:"chatgpt_user_id"`
+	ChatGPTPlanType  string              `json:"chatgpt_plan_type"`
 	UserID           string              `json:"user_id"`
 	Organizations    []OrganizationClaim `json:"organizations"`
 }
@@ -375,6 +376,7 @@ type UserInfo struct {
 	Email            string
 	ChatGPTAccountID string
 	ChatGPTUserID    string
+	PlanType         string
 	UserID           string
 	OrganizationID   string
 	Organizations    []OrganizationClaim
@@ -389,6 +391,7 @@ func (c *IDTokenClaims) GetUserInfo() *UserInfo {
 	if c.OpenAIAuth != nil {
 		info.ChatGPTAccountID = c.OpenAIAuth.ChatGPTAccountID
 		info.ChatGPTUserID = c.OpenAIAuth.ChatGPTUserID
+		info.PlanType = c.OpenAIAuth.ChatGPTPlanType
 		info.UserID = c.OpenAIAuth.UserID
 		info.Organizations = c.OpenAIAuth.Organizations