feat(servers): fix /boot getting filled with kernels.

modules/servers/general/optimise-storage.nix

@@ -0,0 +1,19 @@
+{...}: {
+  nix.optimise = {
+    automatic = true;
+    dates = "weekly";
+  };
+
+  nix.gc = {
+    automatic = true;
+    dates = "weekly";
+    options = "--delete-older-than 10d";
+  };
+
+  nix.extraOptions = ''
+    min-free = ${toString (100 * 1024 * 1024)}
+    max-free = ${toString (1024 * 1024 * 1024)}
+  '';
+
+  boot.loader.grub.configurationLimit = 3;
+}