IPriceService (record/insight/history) wired into PurchaseService.
CompletePurchaseAsync, deriving a PriceObservation per resolved
PurchaseItem. Latest/average/median/lowest/unit-price all computed
in-memory (no MEDIAN in SQLite).

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

IPurchaseService (start/add-item/complete) + Purchase.AddItem/Complete
domain methods. PurchaseController implements a single-page recording
flow mirroring the proven ShoppingList UX pattern. Verified live
end-to-end via dotnet run + curl.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>