Add configDir, corefilePath, and overridesPath constants pointing to /tmp/nexavpn-vpn-dns directory. Update all file path references in writeCorefile and refreshOverrides to use new constants instead of hardcoded /etc/coredns paths.
Add ServiceDNSRecord type and gateway API endpoint to expose active service domain-to-IP mappings. Implement ListServiceDNSRecords repository method querying services table with proxy_ip resolution using effectiveAccessProxyIP helper.
Add vpn-dns microservice built on CoreDNS with periodic sync from backend API. Generate Corefile with configurable upstream DNS servers and hosts plugin for service overrides.
