Document OAuth callback URLs and clarify the errors from API

Dear Dmitry,

I have just deployed Comentario on my server (Debian Trixie up to date) using the comentario_3.4.0_linux_amd64.deb Debian package, with an Apache httpd server configured as a reverse proxy.

I was able to configure the Github OAuth provider, but I can't configure the Gitlab one and I haven't found any information about it in the documentation.

I logged on to Gitlab, then went to my profile page. There, I selected the "Applications" menu item, then clicked on the "Add new application" button.

In the proposed form, I have provided the following information:

• Redirect URI: <BASE_RUL>/oauth/gitlab/callback
• Scopes: read_user

Then, I clicked on the "Save application" button.

On my server, I edited the file /etc/comentario/secrets.yaml and added the "Application ID" as key, and the secret:

idp:
  gitlab:
    key: 6150..71a6
    secret: gloas-485c..c8aa
    # disable: true

After that, I restarted the Comentario service and I activated the Gitlab provider in the configuration of my domain:

Then I tried to authenticate via Gitlab and got the following error:

Can you tell me what I missed?