feat(pxe-boot): mount pxe-images NFS share and symlink to /srv/pxe/http/images

Adds modules/pxe-boot/mount-pxe-images.nix, which mounts
server.sweet.home:/tank/proxmox/pxe-images at /mnt/pxe-images via NFSv4.2
(x-systemd.automount on Proxmox VMs, nofail on LXC containers — same pattern
as docker/mount-data.nix). The pxe-boot build-type now imports this module and
replaces the previous local /srv/pxe/http/images directory rule with an L+
symlink pointing to /mnt/pxe-images, so large images (ISOs, disk images) live
on the NFS share rather than the host's own root disk.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

modules/build-types/pxe-boot.nix

@@ -132,6 +132,7 @@ in
 {
   imports = [
     ../pxe-boot/stage-installer-artifacts.nix
+    ../pxe-boot/mount-pxe-images.nix
   ];
 
   environment.systemPackages = with pkgs; [
@@ -170,7 +171,7 @@ in
     tmpfiles.rules = [
       "d ${pxeRoot} 0755 root root -"
       "d ${httpRoot} 0755 root root -"
-      "d ${httpRoot}/images 0755 root root -"
+      "L+ ${httpRoot}/images - - - - ${vars.nfsShares.proxmoxPxeImages.mountpoint}"
       "d ${httpRoot}/auto-installer 0755 root root -"
       "d ${httpRoot}/nixos-minimal 0755 root root -"
       "d ${httpRoot}/systemrescue 0755 root root -"