refactor(nixos): make ZFS configuration optional

- move ZFS and snapshot manager settings out of the global base
- group snapshot files under common/optional/zfs
- enable the ZFS module on existing ZFS hosts
- make the global tmpfs setting overridable

common/global/default.nix

@@ -17,16 +17,11 @@
     ./nix.nix
     ./programs.nix
     ./ssh.nix
-    ./snapshot_manager.nix
   ];
 
   boot = {
-    tmp.useTmpfs = true;
+    tmp.useTmpfs = lib.mkDefault true;
     kernelPackages = lib.mkDefault pkgs.linuxPackages_6_12;
-    zfs = {
-      package = lib.mkDefault pkgs.zfs_2_4;
-      forceImportRoot = lib.mkDefault false;
-    };
   };
 
   hardware.enableRedistributableFirmware = true;
@@ -50,16 +45,6 @@
 
     # firmware update
     fwupd.enable = true;
-
-    snapshot_manager = {
-      enable = lib.mkDefault true;
-      PYTHONPATH = "${inputs.self}/";
-    };
-
-    zfs = {
-      trim.enable = lib.mkDefault true;
-      autoScrub.enable = lib.mkDefault true;
-    };
   };
 
   powerManagement.powertop.enable = lib.mkDefault true;

common/optional/zfs/default.nix

@@ -0,0 +1,26 @@
+{
+  inputs,
+  lib,
+  pkgs,
+  ...
+}:
+{
+  imports = [ ./snapshot.nix ];
+
+  boot.zfs = {
+    package = lib.mkDefault pkgs.zfs_2_4;
+    forceImportRoot = lib.mkDefault false;
+  };
+
+  services = {
+    snapshot_manager = {
+      enable = lib.mkDefault true;
+      PYTHONPATH = "${inputs.self}/";
+    };
+
+    zfs = {
+      trim.enable = lib.mkDefault true;
+      autoScrub.enable = lib.mkDefault true;
+    };
+  };
+}