When offline_access scope is configured, Pulse now stores and uses
OIDC refresh tokens to automatically extend sessions. Sessions remain
valid as long as the IdP allows token refresh (typically 30-90 days).
Changes:
- Store OIDC tokens (refresh token, expiry, issuer) alongside sessions
- Automatically refresh tokens when access token nears expiry
- Invalidate session if IdP revokes access (forces re-login)
- Add background token refresh with concurrency protection
- Persist OIDC tokens across restarts
Related to #854
