feat(admin): add admin settings panel for OIDC configuration
- Admin UI at /admin.html with settings management interface
- REST API: GET/PUT /api/admin/settings/oidc, POST .../test, GET .../general
- DB-backed settings in auth.admin_settings table (PostgreSQL)
- OIDC auto-discovery from issuer URL (.well-known/openid-configuration)
- Hot-reload: OIDC config changes apply without server restart
- Role-based access: admin-only endpoints with 403 for regular users
- Client secret stored securely, never exposed in GET responses
- Env var override detection shown in admin UI
- Clean architecture: repository trait, PG implementation, service, handler
Dionisio
f60c0df9f9