feat(33-01): add currency conversion service with exchange rate caching

- Create currency.service.ts with frankfurter.app ECB rate fetching
- 24h in-memory cache with stale-serve fallback on fetch failure
- convertPrice() handles EUR-base cross-currency conversion
- CURRENCY_MARKET_MAP maps currencies to market regions
- 12 unit tests covering conversion, rounding, unknowns, and mapping

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/server/services/currency.service.ts

@@ -0,0 +1,112 @@
+export interface ExchangeRates {
+	base: string;
+	date: string;
+	rates: Record<string, number>;
+}
+
+export const CURRENCY_MARKET_MAP: Record<string, string> = {
+	EUR: "EU",
+	USD: "US",
+	GBP: "UK",
+	JPY: "JP",
+	CAD: "CA",
+	AUD: "AU",
+};
+
+export function getMarketForCurrency(currency: string): string {
+	return CURRENCY_MARKET_MAP[currency] ?? currency;
+}
+
+// In-memory cache
+let cachedRates: ExchangeRates | null = null;
+let cacheExpiry = 0;
+
+const CACHE_TTL_MS = 24 * 60 * 60 * 1000; // 24 hours
+const FRANKFURTER_URL = "https://api.frankfurter.app/latest?from=EUR";
+
+export async function getExchangeRates(): Promise<ExchangeRates> {
+	const now = Date.now();
+
+	// Return cached rates if still valid
+	if (cachedRates && now < cacheExpiry) {
+		return cachedRates;
+	}
+
+	try {
+		const response = await fetch(FRANKFURTER_URL);
+		if (!response.ok) {
+			throw new Error(`frankfurter.app returned ${response.status}`);
+		}
+
+		const data = (await response.json()) as {
+			base: string;
+			date: string;
+			rates: Record<string, number>;
+		};
+
+		// Validate response shape
+		if (!data.base || !data.rates || typeof data.rates !== "object") {
+			throw new Error("Invalid exchange rate response shape");
+		}
+
+		// Reject if any rate is negative or zero
+		for (const [currency, rate] of Object.entries(data.rates)) {
+			if (typeof rate !== "number" || rate <= 0) {
+				throw new Error(`Invalid rate for ${currency}: ${rate}`);
+			}
+		}
+
+		// Include base currency in rates for conversion math
+		const rates: ExchangeRates = {
+			base: data.base,
+			date: data.date,
+			rates: { ...data.rates, [data.base]: 1 },
+		};
+
+		cachedRates = rates;
+		cacheExpiry = now + CACHE_TTL_MS;
+
+		return rates;
+	} catch (error) {
+		// Stale-serve: return cached rates if available
+		if (cachedRates) {
+			return cachedRates;
+		}
+		throw error;
+	}
+}
+
+/**
+ * Convert price in cents from one currency to another.
+ * All conversions go through EUR as the base currency.
+ */
+export function convertPrice(
+	cents: number,
+	from: string,
+	to: string,
+	rates: ExchangeRates,
+): number {
+	if (from === to) return cents;
+
+	const fromRate = rates.rates[from];
+	const toRate = rates.rates[to];
+
+	if (fromRate == null || toRate == null) {
+		// Unknown currency — return original
+		return cents;
+	}
+
+	// Convert: from → EUR → to
+	const centsInEur = cents / fromRate;
+	const result = centsInEur * toRate;
+
+	return Math.round(result);
+}
+
+/**
+ * Reset cache — for testing only.
+ */
+export function resetCache(): void {
+	cachedRates = null;
+	cacheExpiry = 0;
+}