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/private/lib/logAccessAudit.ts

@@ -161,8 +161,11 @@ async function getCountryCodeFromIp(ip: string): Promise<string | undefined> {
 
     if (!cachedCountryCode) {
         cachedCountryCode = await getCountryCodeForIp(ip); // do it locally
-        // Cache for longer since IP geolocation doesn't change frequently
-        cache.set(geoIpCacheKey, cachedCountryCode, 300); // 5 minutes
+        // Only cache successful lookups to avoid filling cache with undefined values
+        if (cachedCountryCode) {
+            // Cache for longer since IP geolocation doesn't change frequently
+            cache.set(geoIpCacheKey, cachedCountryCode, 300); // 5 minutes
+        }
     }
 
     return cachedCountryCode;