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

src/infrastructure/repositories/pg/mod.rs

@@ -7,6 +7,7 @@ mod contact_persistence_dto;
 mod favorites_pg_repository;
 mod recent_items_pg_repository;
 mod session_pg_repository;
+mod settings_pg_repository;
 mod transaction_utils;
 mod user_pg_repository;
 
@@ -19,4 +20,5 @@ pub use contact_persistence_dto::*;
 pub use favorites_pg_repository::FavoritesPgRepository;
 pub use recent_items_pg_repository::RecentItemsPgRepository;
 pub use session_pg_repository::SessionPgRepository;
+pub use settings_pg_repository::SettingsPgRepository;
 pub use user_pg_repository::UserPgRepository;