refactor(actualbudget): split service and guest config

guests/actualbudget.nix

@@ -1,23 +1,43 @@
-args@{ my, ... }:
-# Actual Budget in a NixOS guest, with durable state bind-mounted from the host.
-my.guest {
-  name = "actualbudget";
-  interior = {
-    users.users.actual = {
-      isSystemUser = true;
-      uid = 20001;
-      group = "storage";
-    };
-
-    services.actual = {
-      enable = true;
-      openFirewall = true;
-      user = "actual";
-      group = "storage";
-      settings = {
-        port = 5006;
-        dataDir = "/data";
+{
+  config,
+  lib,
+  my,
+  ...
+}:
+let
+  cfg = config.guests.actualbudget;
+  dataDir = "/data";
+in
+{
+  imports = [
+    (my.guest {
+      name = "actualbudget";
+      interior = {
+        modules.services.actualbudget = {
+          enable = true;
+          inherit dataDir;
+        };
       };
+    })
+  ];
+
+  options.guests.actualbudget = {
+    dataPath = lib.mkOption {
+      type = lib.types.nullOr lib.types.str;
+      default = null;
+      example = "/srv/actualbudget";
+      description = "Host path bind-mounted as Actual Budget's data directory.";
     };
   };
-} args
+
+  config = lib.mkIf cfg.enable {
+    assertions = [
+      {
+        assertion = cfg.dataPath != null;
+        message = "guests.actualbudget.dataPath must point at the host directory that stores Actual Budget data.";
+      }
+    ];
+
+    guests.actualbudget.mounts.${dataDir}.hostPath = cfg.dataPath;
+  };
+}