refactor: abstract app user creation via custom options

modules/common/default.nix

@@ -3,6 +3,7 @@
     ./docker.nix
     ./monitoring.nix
     ./nfs.nix
+    ./users.nix
   ];
 
   config = {

modules/common/users.nix

@@ -0,0 +1,37 @@
+{ config, lib, ... }:
+
+{
+  options.homelab.appUsers = lib.mkOption {
+    type = lib.types.attrsOf (lib.types.submodule {
+      options = {
+        uid = lib.mkOption {
+          type = lib.types.int;
+          description = "The user ID for the app user";
+        };
+        group = lib.mkOption {
+          type = lib.types.str;
+          default = config.users.groups.apps.name;
+          description = "The primary group for the app user";
+        };
+        extraGroups = lib.mkOption {
+          type = lib.types.listOf lib.types.str;
+          default = [];
+          description = "Extra groups for the app user";
+        };
+      };
+    });
+    default = {};
+    description = "App users to automatically create with standard homelab options";
+  };
+
+  config = {
+    users.users = lib.mapAttrs (name: cfg: {
+      uid = lib.mkForce cfg.uid;
+      isSystemUser = true;
+      group = cfg.group;
+      extraGroups = cfg.extraGroups;
+      home = "/var/empty";
+      shell = null;
+    }) config.homelab.appUsers;
+  };
+}