Fix: Prevent cache memory leak by adding maxKeys limit and conditional caching

- Add maxKeys limit (10,000) to NodeCache to prevent unbounded memory growth
- Skip caching undefined values when GeoIP/ASN lookups fail (e.g., when MaxMind DB not configured)
- Add periodic cache statistics logging every 5 minutes for monitoring
- Fixes memory leak where cache would grow indefinitely with high request volumes

The maxKeys limit uses LRU eviction, so oldest entries are automatically removed
when the limit is reached. With ~10k requests/day and 5min TTL, 10k keys provides
ample headroom while preventing OOM issues.

Fixes #2120

server/lib/cache.ts

@@ -1,5 +1,20 @@
 import NodeCache from "node-cache";
+import logger from "@server/logger";
 
-export const cache = new NodeCache({ stdTTL: 3600, checkperiod: 120 });
+// Create cache with maxKeys limit to prevent memory leaks
+// With ~10k requests/day and 5min TTL, 10k keys should be more than sufficient
+export const cache = new NodeCache({
+    stdTTL: 3600,
+    checkperiod: 120,
+    maxKeys: 10000
+});
+
+// Log cache statistics periodically for monitoring
+setInterval(() => {
+    const stats = cache.getStats();
+    logger.debug(
+        `Cache stats - Keys: ${stats.keys}, Hits: ${stats.hits}, Misses: ${stats.misses}, Hit rate: ${stats.hits > 0 ? ((stats.hits / (stats.hits + stats.misses)) * 100).toFixed(2) : 0}%`
+    );
+}, 300000); // Every 5 minutes
 
 export default cache;