Vikunja: Scoped API token can mint unrestricted OAuth session credentials

Vikunja is an open-source self-hosted task management platform. In version 2.3.0, a scoped API token limited to the `oauth.authorize` permission can call `POST /api/v1/oauth/authorize`, obtain an OAuth authorization code, and exchange the code at `POST /api/v1/oauth/token` for a normal bearer JSON Web Token (JWT) and refresh token. The resulting credentials are not restricted by the original API token's permissions, allowing access to routes outside its declared scope for the same user. Version 2.4.0 fixes the vulnerability.