Admin-only audit trail with agent actor support

Admins now have an /audit page that shows an append-only trail of actions from the dashboard, MCP, webhooks, workers, and system flows. The trail records a polymorphic actor for each event: a user, a Condukt-driven agent, or a system process, and it is also exposed through the list_audit_activities and get_audit_activity MCP tools. User roles are now persisted as member or admin, and only admins can access the audit trail.