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

hosts/pxe-boot/host.nix

@@ -1,7 +1,16 @@
-_:
+{ vars, ... }:
 
 {
-  networking.hostName = "pxe-boot";
+  networking = {
+    hostName = "pxe-boot";
+    useDHCP = false;
+    interfaces.${vars.lxcLanInterface}.ipv4.addresses = [{
+      address = vars.pxeServerIp;
+      prefixLength = vars.lanPrefixLength;
+    }];
+    defaultGateway = { address = vars.lanGateway; interface = vars.lxcLanInterface; };
+    nameservers = [ vars.domainControllerIp ];
+  };
 
   # Preserved from the pre-refactor `pxe-boot` target — stateVersion must
   # never be bumped on an already-installed machine.