JWT generation for frontend on successful login

## Summary
Implement [JWT](https://www.jwt.io/) generation for the frontend. After successful authentication via USOS OAuth and completion of the background user provisioning, the system must generate a signed JWT to authorize subsequent API requests from the frontend.

## [Acceptance criteria](https://www.atlassian.com/work-management/project-management/acceptance-criteria)
- [ ] A signed JWT is generated immediately after a successful user provisioning in the OAuth callback flow.
- [ ] The JWT payload besides standard fields contains only the internal user id to ensure anonymity and become lightweight.
- [ ] The callback endpoint successfully returns the generated JWT to the frontend client.
- [ ] The backend exposes example endpoint protected against guest access (only logged in users can access it) that successfully validates the generated token when passed in the `Authorization: Bearer` header.
- [ ] When access is not granted appropiate HTTP code and error message is returned from API (that should be documented)
## Notes
important context