PriceController exposes a prices index (sortable by name/price/recency),
per-product price history with manual observation logging, and real
price insight on the product details page, replacing the placeholder
text left from CW-EPIC-04. Completes CW-EPIC-05.

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

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>