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

systems/bob/default.nix

@@ -6,15 +6,16 @@
     "${inputs.self}/users/steve"
     "${inputs.self}/common/global"
     "${inputs.self}/common/optional/docker.nix"
-    "${inputs.self}/common/optional/scanner.nix"
     "${inputs.self}/common/optional/monitoring-agent.nix"
+    "${inputs.self}/common/optional/nvidia.nix"
+    "${inputs.self}/common/optional/scanner.nix"
     "${inputs.self}/common/optional/steam.nix"
     "${inputs.self}/common/optional/syncthing_base.nix"
     "${inputs.self}/common/optional/systemd-boot.nix"
     "${inputs.self}/common/optional/update.nix"
     "${inputs.self}/common/optional/yubikey.nix"
     "${inputs.self}/common/optional/zerotier.nix"
-    "${inputs.self}/common/optional/nvidia.nix"
+    "${inputs.self}/common/optional/zfs"
     ./hardware.nix
     ./syncthing.nix
     ./llms.nix