F-177 — Architecture

Add orders_orders.state_changed_at, initialized from updated_at, indexed for monitored states.
Every state transition updates both updated_at and state_changed_at; non-state edits only update updated_at.
Admin endpoint GET /orders/notifications/stale?hours=24&limit=20: RBAC admin, fixed monitored states, parameterized threshold, total + items.
Header client component polls every 5 minutes, refreshes when opened, closes on Escape/outside click, and links to /orders/:id.
Bell badge caps visual text at 99+; dropdown differentiates pending and shipped.