add aspects/server/ NixOS modules

Server-specific NixOS configuration: boot, nix, tailscale

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

aspects/server/boot.nix

@@ -0,0 +1,10 @@
+# aspects/server/boot.nix
+{ inputs, ... }:
+{
+  flake.modules.nixos.server-boot = { config, lib, pkgs, ... }: {
+    # Import parent aspect for inheritance
+    imports = [ inputs.self.modules.nixos.common-boot ];
+
+    boot.kernelPackages = pkgs.linuxPackages_hardened;
+  };
+}

aspects/server/nix.nix

@@ -0,0 +1,18 @@
+# aspects/server/nix.nix
+{ inputs, ... }:
+{
+  flake.modules.nixos.server-nix = { config, lib, pkgs, ... }: {
+    # Import parent aspect for inheritance
+    imports = [ inputs.self.modules.nixos.common-nix ];
+
+    environment.etc."channels/nixpkgs".source = inputs.nixpkgs-stable.outPath;
+
+    nix = {
+      registry.nixpkgs.flake = inputs.nixpkgs-stable;
+      nixPath = [
+        "nixpkgs=/etc/channels/nixpkgs"
+        "/nix/var/nix/profiles/per-user/root/channels"
+      ];
+    };
+  };
+}

aspects/server/tailscale.nix

@@ -0,0 +1,18 @@
+# aspects/server/tailscale.nix
+{ inputs, ... }:
+{
+  flake.modules.nixos.server-tailscale = { config, lib, pkgs, ... }: {
+    # Import parent aspect for inheritance
+    imports = [ inputs.self.modules.nixos.common-tailscale ];
+
+    services.tailscale = {
+      extraSetFlags = [ "--advertise-exit-node" ];
+      useRoutingFeatures = "server";
+    };
+
+    boot.kernel.sysctl = {
+      "net.ipv4.ip_forward" = 1;
+      "net.ipv6.conf.all.forwarding" = 1;
+    };
+  };
+}