feat(server): add OIDC logout URL override option (#27389)

* feat(server): add OIDC logout URL override option
- Added toggle and field consistent with existing mobile redirect URI override.
- Existing auto-discovery remains default.
- Update tests and docs for new feature.

* fix(server): changes from review for OIDC logout URL override
- Rename 'logoutUri' to 'endSessionEndpoint'
- Remove toggle, just use override if provided
- Moved field in settings UI

server/src/services/auth.service.ts

@@ -455,6 +455,10 @@ export class AuthService extends BaseService {
       return LOGIN_URL;
     }
 
+    if (config.oauth.endSessionEndpoint) {
+      return config.oauth.endSessionEndpoint;
+    }
+
     return (await this.oauthRepository.getLogoutEndpoint(config.oauth)) || LOGIN_URL;
   }