fix(netinfra): kill IPv6 on DNS nodes to stop Pi-hole flapping

Root cause of Uptime Kuma DNS up/down alerts: Pi-hole's upstream config
included Google IPv6 DNS (2001:4860:4860::8888), but netinfra-01 has no
IPv6 internet route. Every forwarded query to the IPv6 upstream failed
with "Network unreachable", causing intermittent DNS resolution
failures every ~8 seconds.

Fix applied to both netinfra-01 and netinfra-02:
- Pi-hole upstream set to 8.8.8.8 only (IPv4); removed 192.168.3.16
  (retired netboot) and 2001:4860:4860::8888 (IPv6 Google DNS)
- IPv6 disabled at kernel level (/etc/sysctl.d/99-disable-ipv6.conf)
- knel.net authoritative resolution unchanged (Technitium via revServers)

Verified: zero IPv6 warnings, zero connection errors, DNS resolving
cleanly from all paths after fix.

[#376]

netinfra/pihole/docker-compose.yml

@@ -16,6 +16,7 @@ services:
       TZ: 'America/Chicago'
       FTLCONF_webserver_api_password: '${PIHOLE_WEB_PASSWORD}'
       FTLCONF_dns_listeningMode: 'all'
+      FTLCONF_dns_upstreams: '["8.8.8.8"]'
     volumes:
       - './etc-pihole:/etc/pihole'
       - './etc-dnsmasq.d:/etc/dnsmasq.d'