Phase 0: gnexus-auth integration infrastructure ...

- Add gnexus/auth-client via Composer vcs
- Thin PSR-18/17 cURL adapter (zero extra deps)
- SessionStateStore, SessionPkceStore, DbTokenStore
- AuthService wrapper: login URL, callback, logout, refresh, me
- UserResolver + PermissionResolver
- AuthController (login, callback, logout, me, refresh)
- WebhookController + WebhookRouter + event handlers
- AuthControllerTrait for endpoint protection
- Migration: 9 tables (users, roles, permissions, overrides, groups, members, group_perms, sessions, audit) + seed data
- Remove old Example_AuthController
- Add GAUTH_* env variables to config and .env.example