perf: cache online API endpoints with Redis-first cache

- Upgrade lib/cache.ts: Redis-first cached() with in-memory fallback
  (was in-memory only, broken across PM2 instances)
- Cache /api/online user list (10s TTL, was uncached per-request)
  eliminates DB query on every poll request
- Add uncached() invalidation helper for write-after-cache patterns
- 0 tsc errors, 583 tests passing

src/app/api/online/route.ts

@@ -5,17 +5,23 @@
 // 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 users = await prisma.user.findMany({
-			where: { online: "1" },
-			select: { username: true, look: true },
-			take: 100,
-		});
+		const users = await cached(
+			"online_users",
+			10_000,
+			async () =>
+				prisma.user.findMany({
+					where: { online: "1" },
+					select: { username: true, look: true },
+					take: 100,
+				}),
+		);
 
 		return apiJson({ users });
 	} catch {