feat(networking): declare static IPs for all NixOS-managed hosts in flake

Adds static IP configuration to every NixOS host in the flake that
has a fixed LAN address, and centralises all network primitives
(IPs, gateway, prefix length, interface names) in variables.nix so
there is one place to update if any of them change.

variables.nix additions:
- lanGateway / lanPrefixLength — LAN gateway and /24 prefix, replacing
  every hardcoded 192.168.2.254 / 24 across host files
- lxcLanInterface / vmLanInterface / vmStorageInterface — NIC names for
  LXC containers (eth0), Proxmox VMs (ens18), and the HA storage NIC
  (ens19), used as attribute keys so changing the name is a one-line edit
- haStoragePrefixLength — /29 for the storage subnet, mirrors haStorageCidr
- Per-host IP variables: nixCacheIp (.224), tailscaleRouterIp (.222),
  torRelayIp (.221), serverIp (.226), dockerIp (.225)

host.nix changes:
- tailscale-router, tor-relay, nix-cache, pxe-boot: useDHCP = false,
  static address on eth0 (lxcLanInterface), struct-form defaultGateway
  (required when using systemd-networkd which LXC containers use)
- server, docker: useDHCP = false, static address on ens18 (vmLanInterface),
  struct-form defaultGateway (works for both scripted networking and networkd)
- ha-server-1, ha-server-2: replace hardcoded 192.168.2.254 / 24 / 29
  with the new variables; no functional change for these hosts

modules/build-types/pxe-boot.nix:
- Domain-controller kickstart template: replace hardcoded 192.168.2.138
  and 192.168.2.254 with vars.domainControllerIp / vars.lanGateway /
  vars.lanPrefixLength / vars.homeDomain so the template stays correct
  if the DC IP or domain is ever changed again

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01ULXzafSDwGhmFGnn3LtDSQ

variables.nix

@@ -4,9 +4,19 @@
   homeDomain = "sweet.home"; # base LAN domain for service subdomains (pve., docker.)
   tailnetDomain = "tail13f623.ts.net"; # Tailscale MagicDNS suffix
   lanCidr = "192.168.2.0/24"; # LAN subnet
-  pxeServerIp = "192.168.2.223"; # pxe-boot host's LAN IP
-  pbsIp = "192.168.2.244"; # Proxmox Backup Server LAN IP
-  domainControllerIp = "192.168.2.253"; # FreeIPA domain controller / primary DNS
+  lanGateway = "192.168.2.254"; # LAN default gateway (router)
+  lanPrefixLength = 24; # LAN subnet prefix length (/24 = 255.255.255.0)
+  lxcLanInterface = "eth0"; # LAN NIC name in LXC containers (set by Proxmox --net0 name=eth0)
+  vmLanInterface = "ens18"; # LAN NIC name in Proxmox VMs (virtio, first NIC)
+  vmStorageInterface = "ens19"; # storage NIC name in HA server VMs (virtio, second NIC on vmbr1)
+  pxeServerIp = "192.168.2.223"; # pxe-boot LXC container LAN IP
+  nixCacheIp = "192.168.2.224"; # nix-cache LXC container LAN IP
+  tailscaleRouterIp = "192.168.2.222"; # tailscale-router LXC container LAN IP
+  torRelayIp = "192.168.2.221"; # tor-relay LXC container LAN IP
+  serverIp = "192.168.2.226"; # server (NFS/ZFS) Proxmox VM LAN IP
+  dockerIp = "192.168.2.225"; # docker Proxmox VM LAN IP
+  pbsIp = "192.168.2.244"; # Proxmox Backup Server LAN IP (not NixOS-managed)
+  domainControllerIp = "192.168.2.253"; # FreeIPA domain controller / primary DNS (not NixOS-managed)
 
   # Cross-host references (LAN hostnames/users other hosts reach over the network)
   nixCacheHost = "nix-cache"; # substituter/remote-builder hostname
@@ -83,6 +93,7 @@
   haServer1StorageIp = "192.168.4.228"; # storage-net IP, node 1 (vmbr1 / ens19)
   haServer2StorageIp = "192.168.4.227"; # storage-net IP, node 2 (vmbr1 / ens19)
   haStorageCidr = "192.168.4.0/29"; # storage subnet — internal to pve1 only
+  haStoragePrefixLength = 29; # storage subnet prefix length (/29)
   haStorageRoot = "/srv/ha-data"; # XFS-over-DRBD mount point on the Active node
   haIscsiIqn = "iqn.2026-01.home.sweet:ha-storage";