DNS Monitor: support custom/authoritative DNS server (custom resolver)#1607

Problem

Currently, the DNS monitor only queries public resolvers. There is no option to specify a custom DNS server to query against.

Use Case

I run my own authoritative nameserver and need to monitor DNS resolution directly against it. This is essential for:

• Self-hosted DNS setups
• GeoIP-based DNS servers
• Any infrastructure where you need to verify what a specific nameserver is returning not what public resolvers have cached

Requested Feature

Add a “Custom resolver / DNS server” field in the DNS monitor configuration.