system: enable zram swap for all hosts

Provides compressed swap in RAM to prevent OOM kills during
nixos-rebuild on low-memory VMs (2GB). Removes duplicate zram
configs from jelly01 and nix-cache01.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

system/default.nix

@@ -12,5 +12,6 @@
     ./pki/root-ca.nix
     ./sshd.nix
     ./vault-secrets.nix
+    ./zram.nix
   ];
 }

system/zram.nix

@@ -0,0 +1,8 @@
+# Compressed swap in RAM
+#
+# Provides overflow memory during Nix builds and upgrades.
+# Prevents OOM kills on low-memory hosts (2GB VMs).
+{ ... }:
+{
+  zramSwap.enable = true;
+}