Price series for one item key across chains and weeks

Every offer sharing one persisted item key (as carried by a search group) whose validity touches the last days days, oldest first, with lowest/median context. summary is computed from the CAMPAIGN rows only — these are ADVERTISED leaflet prices, never a normal or shelf price. The series also carries regular rows (everyday shelf prices, Salling only); use each point's price_kind to tell them apart. summary is null when the key has no campaign offer in the window, even if it has shelf prices; that is not an error.