feat: add server-side TTL cache + in-flight dedup to request handler

10s default TTL deduplicates concurrent requests from multiple clients
polling the same integration. 83 users hitting the same widget every
30s now produce ~6 upstream calls/min instead of ~166.

packages/request-handler/src/lib/request-handler.ts

@@ -1,14 +1,44 @@
 interface Options<TData, TInput extends Record<string, unknown>> {
   requestAsync: (input: TInput) => Promise<TData>;
+  cacheTtlMs?: number;
 }
 
+type CacheEntry<TData> = { data: TData; timestamp: Date; expiresAt: number };
+
+// ponytail: module-level cache keyed by JSON(input). Ceiling: unbounded keys if inputs are high-cardinality; upgrade to LRU if needed.
+const cache = new Map<string, CacheEntry<unknown>>();
+const inflight = new Map<string, Promise<CacheEntry<unknown>>>();
+
+const DEFAULT_TTL_MS = 10_000;
+
 export const createRequestHandler = <TData, TInput extends Record<string, unknown>>(
   options: Options<TData, TInput>,
 ) => ({
   handler: (input: TInput) => ({
     async getDataAsync(): Promise<{ data: TData; timestamp: Date }> {
-      const data = await options.requestAsync(input);
-      return { data, timestamp: new Date() };
+      const ttl = options.cacheTtlMs ?? DEFAULT_TTL_MS;
+      const key = JSON.stringify(input);
+
+      const cached = cache.get(key) as CacheEntry<TData> | undefined;
+      if (cached && Date.now() < cached.expiresAt) {
+        return { data: cached.data, timestamp: cached.timestamp };
+      }
+
+      const existing = inflight.get(key);
+      if (existing) return existing as Promise<CacheEntry<TData>>;
+
+      const promise = options.requestAsync(input).then((data) => {
+        const entry: CacheEntry<TData> = { data, timestamp: new Date(), expiresAt: Date.now() + ttl };
+        cache.set(key, entry as CacheEntry<unknown>);
+        inflight.delete(key);
+        return entry;
+      }).catch((err) => {
+        inflight.delete(key);
+        throw err;
+      });
+
+      inflight.set(key, promise as Promise<CacheEntry<unknown>>);
+      return promise;
     },
   }),
 });