Built from the cost control's own change history; changes to its linked alert do not count. createdBy and createdAt are null for an item created before change history began; lastChangedBy and lastChangedAt are null when nothing changed since then; historyStartsAt is null when the item has no recorded history. Authors are masked: a Revenium staff member shows as Revenium Support, a change made by the system or recorded when change history began shows as Revenium (system), and an API key shows its name and owner. A deleted item's history stays readable. An unknown, malformed or unreadable id answers 404.