- Add InviteToken model and Alembic migration (core.invite_tokens)
- Add invites module: DTOs, service (create/validate/consume), routes
- Register invites router in core router
- Extend auth register endpoint to support invite_token flow:
- Validate local token, create user via Hub admin API (service account),
handle existing user (link instead of duplicate), create UserTenant, consume token
- Add GET /auth/register/check endpoint to validate token without consuming
- Add HUB_ADMIN_EMAIL, HUB_ADMIN_PASSWORD, APP_PUBLIC_URL to config
- Frontend: add invite() method and types to users.ts API client
- Frontend: add invite dialog with real roles dropdown to users page
- Frontend: update register page to handle invite flow and user-exists case
