feat(domain): add currency conversion

Missing rates throw MissingRateError rather than silently treating the
value as base currency.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/domain/fx.test.ts

@@ -0,0 +1,43 @@
+import { test, expect, describe } from "bun:test";
+import { toBase, fromBase, MissingRateError } from "./fx.ts";
+import type { FxRate } from "./types.ts";
+
+const rates: FxRate[] = [
+  { currency: "EUR", baseUnitsPerOne: 7.46, updatedAt: "2026-09-01", source: "manual" },
+  { currency: "USD", baseUnitsPerOne: 6.9, updatedAt: "2026-09-01", source: "manual" },
+];
+
+describe("toBase", () => {
+  test("passes base-currency values through untouched", () => {
+    expect(toBase({ amount: 100, currency: "DKK" }, "DKK", rates)).toBe(100);
+  });
+
+  test("converts using baseUnitsPerOne", () => {
+    expect(toBase({ amount: 10, currency: "EUR" }, "DKK", rates)).toBe(74.6);
+  });
+
+  test("rounds the conversion result", () => {
+    expect(toBase({ amount: 3.33, currency: "USD" }, "DKK", rates)).toBe(22.98);
+  });
+
+  test("throws MissingRateError naming the currency", () => {
+    expect(() => toBase({ amount: 1, currency: "USD" }, "DKK", [])).toThrow(
+      MissingRateError,
+    );
+    try {
+      toBase({ amount: 1, currency: "USD" }, "DKK", []);
+    } catch (error) {
+      expect((error as MissingRateError).currency).toBe("USD");
+    }
+  });
+});
+
+describe("fromBase", () => {
+  test("passes base through untouched", () => {
+    expect(fromBase(100, "DKK", "DKK", rates)).toBe(100);
+  });
+
+  test("inverts the conversion", () => {
+    expect(fromBase(74.6, "EUR", "DKK", rates)).toBe(10);
+  });
+});

src/domain/fx.ts

@@ -0,0 +1,38 @@
+import { roundMoney } from "./money.ts";
+import type { Currency, FxRate, Money } from "./types.ts";
+
+export class MissingRateError extends Error {
+  constructor(public readonly currency: Currency) {
+    super(`No exchange rate configured for ${currency}`);
+    this.name = "MissingRateError";
+  }
+}
+
+function rateFor(currency: Currency, rates: FxRate[]): number {
+  const found = rates.find((r) => r.currency === currency);
+  if (!found) throw new MissingRateError(currency);
+  return found.baseUnitsPerOne;
+}
+
+/** Converts a Money value into the base currency, rounded. */
+export function toBase(
+  value: Money,
+  baseCurrency: Currency,
+  rates: FxRate[],
+): number {
+  if (value.currency === baseCurrency) {
+    return roundMoney(value.amount, baseCurrency);
+  }
+  return roundMoney(value.amount * rateFor(value.currency, rates), baseCurrency);
+}
+
+/** Converts a base-currency amount into `target`, rounded. */
+export function fromBase(
+  amount: number,
+  target: Currency,
+  baseCurrency: Currency,
+  rates: FxRate[],
+): number {
+  if (target === baseCurrency) return roundMoney(amount, target);
+  return roundMoney(amount / rateFor(target, rates), target);
+}