feat: implement user invitation system

- 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

backend/api/v1/modules/core/router.py

@@ -1,4 +1,5 @@
 from .auth.routes import router as auth_router
+from .invites.routes import router as invites_router
 from .licenses.routes import router as licenses_router
 from .permissions.routes import router as permissions_router
 from .tenants.routes import router as tenants_router
@@ -12,6 +13,7 @@ from fastapi import APIRouter
 router = APIRouter()
 
 router.include_router(auth_router)
+router.include_router(invites_router, prefix="/core", tags=["core / invites"])
 router.include_router(tenants_router, prefix="/core", tags=["core / tenants"])
 router.include_router(user_tenant_router, prefix="/core", tags=["core / user-tenants"])
 router.include_router(users_router, prefix="/core", tags=["core / users"])