clanModules/state-version: init state-version

This adds a clanModule that will initialize the `system.stateVersion`
upon deploying the machine for the first time.

If the machine is completely reinstalled, then the state version might be
regenerated. But care should be taken if rebuilding from backups.

clanModules/flake-module.nix

@@ -21,6 +21,7 @@
     postgresql = ./postgresql;
     root-password = ./root-password;
     single-disk = ./single-disk;
+    state-version = ./state-version;
     sshd = ./sshd;
     static-hosts = ./static-hosts;
     sunshine = ./sunshine;