- http.js: add credentials: 'include' to fetch() so cookies are sent
  with every API request (fixes /auth/me returning 401 after callback)
- AuthControllerTrait::resolve_user(): ensure session_start() before
  reading
- AuthController::callback(): ensure session_start() before writing

- AuthService::handleCallback(): defer DB token persistence until
  local user ID is resolved; store temp token data in session
- UserResolver: fix lastInsertId() method name, remove avatarUrl()
  call (use profile array instead)

This fixes the loop where user was redirected back to /login after
successful OAuth callback because PHP session cookie was not sent
with the /auth/me request.

- 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