Auto-detect NS_SERVER_IP at API boot

[rdemeritt]

Problem

NS_SERVER_IP is captured once by scripts/setup.sh. If the host LAN IP later changes (DHCP lease, network move, new interface), Technitium A records become stale silently. New site provisioning writes wrong addresses; existing zones drift.

Proposal

At API boot:

• Resolve primary non-loopback interface IP via os.networkInterfaces()
• Compare to NS_SERVER_IP from env
• Mismatch → log loud warning, surface in dashboard health panel
• Optional: support NS_SERVER_IP=auto to bypass env override and always use detected value

~25 LOC + unit test.

Acceptance

• API boot logs detected interface IP
• Mismatch with env → warning log + dashboard banner
• NS_SERVER_IP=auto writes detected IP into DNS A records
• Existing fixed-IP behavior preserved when env is a real IP that matches an interface

Out of scope

• Active zone-record reconciliation when IP changes mid-uptime (separate enhancement)