Add Redis-backed auth sessions

[gabor-boros]

Description

Implement Redis-backed sessions for authentication to prevent repeated logins during OAuth2 flows.

Currently, the authentication service does not use sessions. This means that in any non-credentials OAuth2 flow, visiting the login page again forces the user to log in again.

Using Redis-backed sessions will allow the system to:

• Maintain user session state.
• Avoid unnecessary re-authentication.
• Improve user experience in multi-step authentication processes.

Tasks

1. Add Redis as the session storage backend for authentication.
2. Integrate session management into the authentication service.
3. Ensure that:
   • OAuth2 flows (non-credentials) maintain session state.
   • Returning to the login page during an active session does not require re-login.
4. Configure session expiration and cleanup policies in Redis.
5. Test:
   • Session creation and persistence.
   • Session expiration behavior.
   • Flow behavior across multiple login page visits.