Switch from email-based OIDC matching to the stable `sub` claim.
Existing users are migrated by email on first login, new users are
auto-enrolled from OIDC claims, and admin status is synced from the
IdP's groups claim. Also expose oidc_sub on the admin edit-user page.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
