feat(api): add /api/convert endpoint with shared rates cache and query validation

lib/server/rates-cache.ts

@@ -0,0 +1,40 @@
+import { fetchUnifiedRates } from "@/lib/api/normalize";
+import type { RatesResponse } from "@/lib/rates";
+
+export const RATES_CACHE_TTL_MS = 300_000;
+export const RATES_CACHE_CONTROL_VALUE =
+  "s-maxage=300, stale-while-revalidate=1800";
+
+let cachedRates: RatesResponse | null = null;
+let cacheTimestamp = 0;
+let inFlightRequest: Promise<RatesResponse> | null = null;
+
+export async function getRatesWithCache(): Promise<RatesResponse> {
+  const now = Date.now();
+  const hasFreshCache = cachedRates && now - cacheTimestamp < RATES_CACHE_TTL_MS;
+
+  if (hasFreshCache && cachedRates) {
+    return cachedRates;
+  }
+
+  if (inFlightRequest) {
+    return inFlightRequest;
+  }
+
+  inFlightRequest = (async () => {
+    const freshRates = await fetchUnifiedRates();
+    cachedRates = freshRates;
+    cacheTimestamp = Date.now();
+    return freshRates;
+  })();
+
+  try {
+    return await inFlightRequest;
+  } finally {
+    inFlightRequest = null;
+  }
+}
+
+export function getLastCachedRates(): RatesResponse | null {
+  return cachedRates;
+}