fix: handle IPs, avoid wrong parent-domain lookups, cache whois results

- Bare IP addresses and non-domain hosts (localhost, IPv6) now return a
  null expiry instead of throwing, so they no longer spam error logs.
- Extract the registrable domain with tldts instead of walking up
  hostname labels, so a subdomain can never accidentally return its
  parent domain's expiry date.
- Cache WHOIS lookups per registrable domain (1h positive, 10min
  negative) so visiting details pages does not re-query the registry
  every time.

Addresses review feedback on #3834.

server/src/api/controllers/monitorController.ts

@@ -82,7 +82,7 @@ class MonitorController implements IMonitorController {
 			msg: "Domain expiry retrieved successfully",
 			data: {
 				domain: domainExpiry.domain,
-				expiryDate: new Date(domainExpiry.expiryDate),
+				expiryDate: domainExpiry.expiryDate === null ? null : new Date(domainExpiry.expiryDate),
 			},
 		});
 	});