Sharepoint - List all subsites with REST API

Finally I changed the code a little :

 url: "/portal/_api/web/webs/?$select=title,ServerRelativeUrl",

And also I gave additionnal permission to my user :

Browse Directories

And everything works...

Update 2018: I found another way

url: '/portal/_api/web/GetSubwebs(nWebTemplateFilter=-1,nConfigurationFilter=-1)',

This link should give you all sites and subsites:

https://yoursharepointsite.com/_api/search/query?querytext='(contentclass:STS_Site) (contentclass:STS_Web)'&trimduplicates=false&rowlimit=5000&selectproperties='Title,Url,Path,ParentLink'

You may need to handle duplicates if they come.