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

app/api/rates/route.ts

@@ -1,42 +1,12 @@
 import { NextResponse } from "next/server";
-import { fetchUnifiedRates } from "@/lib/api/normalize";
-import type { RatesResponse } from "@/lib/rates";
-
-const CACHE_TTL_MS = 300_000;
-const CACHE_CONTROL_VALUE = "s-maxage=300, stale-while-revalidate=1800";
-
-let cachedRates: RatesResponse | null = null;
-let cacheTimestamp = 0;
-let inFlightRequest: Promise<RatesResponse> | null = null;
+import {
+  getLastCachedRates,
+  getRatesWithCache,
+  RATES_CACHE_CONTROL_VALUE,
+} from "@/lib/server/rates-cache";
 
 export const revalidate = 300;
 
-async function getRatesWithCache(): Promise<RatesResponse> {
-  const now = Date.now();
-  const hasFreshCache = cachedRates && now - cacheTimestamp < 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 async function GET() {
   try {
     const data = await getRatesWithCache();
@@ -44,15 +14,17 @@ export async function GET() {
     return NextResponse.json(data, {
       status: 200,
       headers: {
-        "Cache-Control": CACHE_CONTROL_VALUE
+        "Cache-Control": RATES_CACHE_CONTROL_VALUE
       }
     });
   } catch (error) {
+    const cachedRates = getLastCachedRates();
+
     if (cachedRates) {
       return NextResponse.json(cachedRates, {
         status: 200,
         headers: {
-          "Cache-Control": CACHE_CONTROL_VALUE,
+          "Cache-Control": RATES_CACHE_CONTROL_VALUE,
           "X-Cache-Fallback": "stale-on-error"
         }
       });