feat(CRIT-010): show market prices on county pages alongside premium dealers

Add OilPrice type and oilPrices.getByCounty() API client method. County
page now fetches both premium listings and scraped oil prices in parallel,
displaying them in separate sections with appropriate table/card views.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/lib/api/index.ts

@@ -1,3 +1,3 @@
 // API Client exports
-export { api, authApi, companyApi, listingApi, listingsApi, stateApi, categoriesApi } from './client';
+export { api, authApi, companyApi, listingApi, listingsApi, oilPricesApi, stateApi, categoriesApi } from './client';
 export * from './types';