Step 5: Update exchange price endpoint to use new pricing config

- Update ExchangeConfigResponse schema with direction-specific fields
- Remove premium_percentage from PriceResponse (now in config)
- Update price endpoint to load pricing config from database
- Update frontend to use direction-specific min/max and calculate premium
- Update tests to seed pricing config
- Add logic to clamp amount when direction changes

frontend/app/exchange/components/PriceDisplay.tsx

@@ -13,6 +13,7 @@ interface PriceDisplayProps {
   lastUpdate: Date | null;
   direction: "buy" | "sell";
   agreedPrice: number;
+  premiumPercent: number;
 }
 
 /**
@@ -94,10 +95,10 @@ export function PriceDisplay({
   lastUpdate,
   direction,
   agreedPrice,
+  premiumPercent,
 }: PriceDisplayProps) {
   const t = useTranslation("exchange");
   const marketPrice = priceData?.price?.market_price ?? 0;
-  const premiumPercent = priceData?.price?.premium_percentage ?? 5;
   const isPriceStale = priceData?.price?.is_stale ?? false;
 
   return (