#644 Tenant Onboarding Workflow

_system.tenant.onboard workflow for new customer tenants. Workflow runs on _platform tenant, internal queue. Triggered when platform admin creates new tenant via API. Steps: 1. internal.tenant.init_rbac - Initialize RBAC roles 2. internal.tenant.create_admin - Create admin user 3. internal.tenant.install_apps - Install pre-installed apps 4. internal.tenant.create_rate_limits - Create default rate limits 5. internal.tenant.mark_bootstrapped - Update status to completed 6. Fork: Submit _system.tenant.welcome workflow Input: tenant_id, admin_email, plan_type (optional) Parent: #639