backwards: Add agenix module

hosts/backwards/modules/age/default.nix

@@ -0,0 +1,14 @@
+{ inputs, pkgs, ... }:
+{
+  imports = [ inputs.agenix.nixosModules.default ];
+
+  config = {
+    age = {
+      identityPaths = [ "/etc/ssh/backwards" ];
+    };
+
+    environment.systemPackages = [
+      inputs.agenix.packages."${pkgs.system}".default
+    ];
+  };
+}