system: add kanidm PAM/NSS client module

Add homelab.kanidm.enable option for central authentication via Kanidm.
The module configures:
- PAM/NSS integration with kanidm-unixd
- Client connection to auth.home.2rjus.net
- Login authorization for ssh-users group

Enable on testvm01-03 for testing.

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

system/kanidm-client.nix

@@ -0,0 +1,36 @@
+{ lib, config, pkgs, ... }:
+let
+  cfg = config.homelab.kanidm;
+in
+{
+  options.homelab.kanidm = {
+    enable = lib.mkEnableOption "Kanidm PAM/NSS client for central authentication";
+
+    server = lib.mkOption {
+      type = lib.types.str;
+      default = "https://auth.home.2rjus.net";
+      description = "URI of the Kanidm server";
+    };
+
+    allowedLoginGroups = lib.mkOption {
+      type = lib.types.listOf lib.types.str;
+      default = [ "ssh-users" ];
+      description = "Groups allowed to log in via PAM";
+    };
+  };
+
+  config = lib.mkIf cfg.enable {
+    services.kanidm = {
+      package = pkgs.kanidm_1_8;
+      enablePam = true;
+
+      clientSettings = {
+        uri = cfg.server;
+      };
+
+      unixSettings = {
+        pam_allowed_login_groups = cfg.allowedLoginGroups;
+      };
+    };
+  };
+}