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/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;
+    };
+  };
+}