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>

tests/services/currency.service.test.ts

@@ -0,0 +1,87 @@
+import { describe, expect, it } from "bun:test";
+import {
+	CURRENCY_MARKET_MAP,
+	type ExchangeRates,
+	convertPrice,
+	getMarketForCurrency,
+} from "../../src/server/services/currency.service";
+
+const mockRates: ExchangeRates = {
+	base: "EUR",
+	date: "2026-04-13",
+	rates: {
+		EUR: 1,
+		USD: 1.08,
+		GBP: 0.86,
+		JPY: 163.5,
+		CAD: 1.47,
+		AUD: 1.65,
+	},
+};
+
+describe("convertPrice", () => {
+	it("returns same amount when from === to", () => {
+		expect(convertPrice(1000, "EUR", "EUR", mockRates)).toBe(1000);
+		expect(convertPrice(2500, "USD", "USD", mockRates)).toBe(2500);
+	});
+
+	it("converts EUR to USD correctly", () => {
+		// 1000 EUR cents * 1.08 = 1080 USD cents
+		expect(convertPrice(1000, "EUR", "USD", mockRates)).toBe(1080);
+	});
+
+	it("converts USD to EUR correctly", () => {
+		// 1080 USD cents / 1.08 * 1 = 1000 EUR cents
+		expect(convertPrice(1080, "USD", "EUR", mockRates)).toBe(1000);
+	});
+
+	it("converts between non-EUR currencies via EUR base", () => {
+		// 1000 USD cents → EUR: 1000 / 1.08 = 925.926 → GBP: 925.926 * 0.86 = 796.296 → rounded 796
+		expect(convertPrice(1000, "USD", "GBP", mockRates)).toBe(796);
+	});
+
+	it("rounds to nearest integer", () => {
+		// 100 EUR cents * 1.08 = 108 USD cents (exact)
+		expect(convertPrice(100, "EUR", "USD", mockRates)).toBe(108);
+		// 100 EUR cents * 0.86 = 86 GBP cents (exact)
+		expect(convertPrice(100, "EUR", "GBP", mockRates)).toBe(86);
+	});
+
+	it("returns original for unknown currency", () => {
+		expect(convertPrice(1000, "EUR", "XYZ", mockRates)).toBe(1000);
+		expect(convertPrice(1000, "XYZ", "EUR", mockRates)).toBe(1000);
+	});
+});
+
+describe("CURRENCY_MARKET_MAP", () => {
+	it("maps EUR to EU", () => {
+		expect(CURRENCY_MARKET_MAP.EUR).toBe("EU");
+	});
+
+	it("maps USD to US", () => {
+		expect(CURRENCY_MARKET_MAP.USD).toBe("US");
+	});
+
+	it("maps GBP to UK", () => {
+		expect(CURRENCY_MARKET_MAP.GBP).toBe("UK");
+	});
+
+	it("maps all 6 supported currencies", () => {
+		expect(Object.keys(CURRENCY_MARKET_MAP)).toHaveLength(6);
+		expect(CURRENCY_MARKET_MAP.JPY).toBe("JP");
+		expect(CURRENCY_MARKET_MAP.CAD).toBe("CA");
+		expect(CURRENCY_MARKET_MAP.AUD).toBe("AU");
+	});
+});
+
+describe("getMarketForCurrency", () => {
+	it("returns mapped market for known currency", () => {
+		expect(getMarketForCurrency("EUR")).toBe("EU");
+		expect(getMarketForCurrency("USD")).toBe("US");
+		expect(getMarketForCurrency("GBP")).toBe("UK");
+	});
+
+	it("returns currency itself for unknown currency", () => {
+		expect(getMarketForCurrency("XYZ")).toBe("XYZ");
+	});
+});