Add in-memory caching for online count, enable compression, and add staleTimes for router cache

src/components/site-header.tsx

@@ -1,6 +1,7 @@
 import { getTranslations } from "next-intl/server";
 import Link from "next/link";
 import { auth } from "@/lib/auth";
+import { cached } from "@/lib/cache";
 import { prisma } from "@/lib/prisma";
 import { siteSettings } from "@/lib/services/site-settings";
 
@@ -15,7 +16,9 @@ export async function SiteHeader() {
 
   let online = 0;
   try {
-    online = await prisma.user.count({ where: { online: "1" } });
+    online = await cached("online_count", 10_000, () =>
+      prisma.user.count({ where: { online: "1" } }),
+    );
   } catch {
     online = 0;
   }