feat(antigravity): 自动设置隐私并支持后台手动重试

新增 Antigravity OAuth 隐私设置能力，在账号创建、刷新、导入和后台
Token 刷新路径自动调用 setUserSettings + fetchUserInfo 关闭遥测；
持久化后同步内存 Extra，错误处理改为日志记录。

Made-with: Cursor

backend/internal/server/routes/admin.go

@@ -257,6 +257,7 @@ func registerAccountRoutes(admin *gin.RouterGroup, h *handler.Handlers) {
 		accounts.POST("/:id/test", h.Admin.Account.Test)
 		accounts.POST("/:id/recover-state", h.Admin.Account.RecoverState)
 		accounts.POST("/:id/refresh", h.Admin.Account.Refresh)
+		accounts.POST("/:id/set-privacy", h.Admin.Account.SetPrivacy)
 		accounts.POST("/:id/refresh-tier", h.Admin.Account.RefreshTier)
 		accounts.GET("/:id/stats", h.Admin.Account.GetStats)
 		accounts.POST("/:id/clear-error", h.Admin.Account.ClearError)