Restructure networking configuration to per-machine modules

- Move networking configs to modules/network/ directory
- Create network-<machine-name>.nix files for each machine
- Add common.nix for shared networking configuration
- Update import paths in machine configurations
- Reduce duplication by using common networking settings

Network modules:
- modules/network/common.nix: Shared settings (nftables, SSH, tailscale)
- modules/network/network-congenital-optimist.nix: Workstation specific
- modules/network/network-sleeper-service.nix: File server specific

machines/sleeper-service/configuration.nix

@@ -1,6 +1,7 @@
 { config, pkgs, inputs, unstable, ... }: {
   imports = [
     ./hardware-configuration.nix
+    ../../modules/network/network-sleeper-service.nix
   ];
 
   # Boot configuration
@@ -11,12 +12,6 @@
     devices = [ "nodev" ];
   };
 
-  # Network configuration
-  networking.hostName = "sleeper-service";
-  networking.networkmanager.enable = true;
-  services.tailscale.enable = true;
-  networking.firewall.enable = true;
-
   # Time and locale
   time.timeZone = "Europe/Oslo";
   i18n.defaultLocale = "en_US.UTF-8";
@@ -30,15 +25,6 @@
   # Enable unfree packages
   nixpkgs.config.allowUnfree = true;
 
-  # SSH access (headless server)
-  services.openssh = {
-    enable = true;
-    settings = {
-      PermitRootLogin = "no";
-      PasswordAuthentication = false;
-    };
-  };
-
   # Basic system packages
   environment.systemPackages = with pkgs; [
     wget