feat: add provider usage tracking

src/gateway/server-methods/usage.ts

@@ -0,0 +1,10 @@
+import { loadProviderUsageSummary } from "../../infra/provider-usage.js";
+import type { GatewayRequestHandlers } from "./types.js";
+
+export const usageHandlers: GatewayRequestHandlers = {
+  "usage.status": async ({ respond }) => {
+    const summary = await loadProviderUsageSummary();
+    respond(true, summary, undefined);
+  },
+};
+