fix: cache usage cost summary

src/gateway/server-methods/usage.ts

@@ -1,16 +1,78 @@
 import { loadConfig } from "../../config/config.js";
+import type { CostUsageSummary } from "../../infra/session-cost-usage.js";
 import { loadCostUsageSummary } from "../../infra/session-cost-usage.js";
 import { loadProviderUsageSummary } from "../../infra/provider-usage.js";
 import type { GatewayRequestHandlers } from "./types.js";
 
+const COST_USAGE_CACHE_TTL_MS = 30_000;
+
+type CostUsageCacheEntry = {
+  summary?: CostUsageSummary;
+  updatedAt?: number;
+  inFlight?: Promise<CostUsageSummary>;
+};
+
+const costUsageCache = new Map<number, CostUsageCacheEntry>();
+
+const parseDays = (raw: unknown): number => {
+  if (typeof raw === "number" && Number.isFinite(raw)) return Math.floor(raw);
+  if (typeof raw === "string" && raw.trim() !== "") {
+    const parsed = Number(raw);
+    if (Number.isFinite(parsed)) return Math.floor(parsed);
+  }
+  return 30;
+};
+
+async function loadCostUsageSummaryCached(params: {
+  days: number;
+  config: ReturnType<typeof loadConfig>;
+}): Promise<CostUsageSummary> {
+  const days = Math.max(1, params.days);
+  const now = Date.now();
+  const cached = costUsageCache.get(days);
+  if (cached?.summary && cached.updatedAt && now - cached.updatedAt < COST_USAGE_CACHE_TTL_MS) {
+    return cached.summary;
+  }
+
+  if (cached?.inFlight) {
+    if (cached.summary) return cached.summary;
+    return await cached.inFlight;
+  }
+
+  const entry: CostUsageCacheEntry = cached ?? {};
+  const inFlight = loadCostUsageSummary({ days, config: params.config })
+    .then((summary) => {
+      costUsageCache.set(days, { summary, updatedAt: Date.now() });
+      return summary;
+    })
+    .catch((err) => {
+      if (entry.summary) return entry.summary;
+      throw err;
+    })
+    .finally(() => {
+      const current = costUsageCache.get(days);
+      if (current?.inFlight === inFlight) {
+        current.inFlight = undefined;
+        costUsageCache.set(days, current);
+      }
+    });
+
+  entry.inFlight = inFlight;
+  costUsageCache.set(days, entry);
+
+  if (entry.summary) return entry.summary;
+  return await inFlight;
+}
+
 export const usageHandlers: GatewayRequestHandlers = {
   "usage.status": async ({ respond }) => {
     const summary = await loadProviderUsageSummary();
     respond(true, summary, undefined);
   },
-  "usage.cost": async ({ respond }) => {
+  "usage.cost": async ({ respond, params }) => {
     const config = loadConfig();
-    const summary = await loadCostUsageSummary({ days: 30, config });
+    const days = parseDays(params?.days);
+    const summary = await loadCostUsageSummaryCached({ days, config });
     respond(true, summary, undefined);
   },
 };