Foundation for cycle-time analytics, lead-time stats, the Timeline view, and stale-item alerts. New post_status_history table: id, post_id, from_status_id, to_status_id, changed_by, changed_at. Populated by an AFTER UPDATE trigger on posts when status_id changes. RLS: read-only to project members.

Unblocks immediate analytics features: median request to shipped, time-in-status, and a public "shipped 23 user requests in 90 days" stat for the marketing surface. No UI in this card - strictly the table + trigger + a tiny query helper. UI cards reference this one as a dependency.