Describe the pull request

This PR adds support for a CUSTOM_LOGOUT_URL configuration under the [auth] section.

Problem: When Gogs is used behind a Reverse Proxy with SSO (like Authelia, Cloudflare Access, or Keycloak), clicking "Sign Out" only clears the Gogs session. The user remains logged into the SSO provider.

Solution: Administrators can now set CUSTOM_LOGOUT_URL to redirect users to the SSO provider's logout endpoint immediately after the Gogs session is destroyed.

Link to the issue: closes #8077

Checklist

• I agree to follow the Code of Conduct by submitting this pull request.
• I have read and acknowledge the Contributing guide.
• I have added test cases to cover the new code or have provided the test plan.

Test plan

1. Add the following to custom/conf/app.ini:

   [auth]
   CUSTOM_LOGOUT_URL = [https://google.com](https://google.com)