Refactor OAuth2 authentication flow for enterprise SSO providers

Currently, the authentication service handles SSO tokens synchronously, causing timeouts when enterprise providers (Okta, Azure AD) have high latency. We need to decouple the token validation process.

This task involves:

• Migrating the validation logic to a background worker queue.
• Implementing a polling mechanism for the frontend client.
• Updating the UserSession schema to support pending states.

Please ensure this is backward compatible with the existing JWT implementation.