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

src/app/api/online/count/route.ts

@@ -4,16 +4,16 @@
 // { count: 0 } (never 500) on DB failure.
 
 import { apiJson } from "@/lib/api";
+import { cached } from "@/lib/cache";
 import { prisma } from "@/lib/prisma";
 
 export const dynamic = "force-dynamic";
 
 export async function GET(_req: Request) {
   try {
-    const count = await prisma.user.count({
-      where: { online: "1" },
-    });
-
+    const count = await cached("online_count", 10_000, () =>
+      prisma.user.count({ where: { online: "1" } }),
+    );
     return apiJson({ count });
   } catch {
     return apiJson({ count: 0 });