fix: enable cloud-init networking in NixOS template

Freshly recreated VMs were reachable but did not accept the injected SSH key, which indicates Proxmox cloud-init settings were not being applied. Enable cloud-init and cloud-init network handling in the base template so static IPs, hostname, ciuser, and SSH keys take effect on first boot.

nixos/template-base/configuration.nix

@@ -10,7 +10,8 @@ in
       ./hardware-configuration.nix;
 
   networking.hostName = "k8s-base-template";
-  networking.useDHCP = lib.mkDefault true;
+  networking.useDHCP = false;
+  networking.useNetworkd = true;
   networking.nameservers = [ "1.1.1.1" "8.8.8.8" ];
 
   boot.loader.systemd-boot.enable = lib.mkForce false;
@@ -20,6 +21,8 @@ in
   };
 
   services.qemuGuest.enable = true;
+  services.cloud-init.enable = true;
+  services.cloud-init.network.enable = true;
   services.openssh.enable = true;
   services.openssh.settings = {
     PasswordAuthentication = false;