feat(mx): add pg_upgrade script for PostgreSQL 14 → 16 migration

Temporary upgrade script following the official NixOS procedure.
Run `upgrade-pg-cluster --jobs 4 --link` on the server, then switch
the package to postgresql_16 and remove the script.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

systems/x86_64-linux/mx/postgresql.nix

@@ -8,4 +8,33 @@
   services.postgresql = {
     package = pkgs.postgresql_14;
   };
+
+  # Temporary: upgrade script for PostgreSQL 14 → 16 migration.
+  # After upgrading, change package above to postgresql_16 and remove this block.
+  environment.systemPackages = [
+    (
+      let
+        newPostgres = pkgs.postgresql_16;
+        cfg = config.services.postgresql;
+      in
+      pkgs.writeScriptBin "upgrade-pg-cluster" ''
+        set -eux
+        systemctl stop postgresql
+
+        export NEWDATA="/var/lib/postgresql/${newPostgres.psqlSchema}"
+        export NEWBIN="${newPostgres}/bin"
+        export OLDDATA="${cfg.dataDir}"
+        export OLDBIN="${cfg.finalPackage}/bin"
+
+        install -d -m 0700 -o postgres -g postgres "$NEWDATA"
+        cd "$NEWDATA"
+        sudo -u postgres "$NEWBIN/initdb" -D "$NEWDATA" ${lib.escapeShellArgs cfg.initdbArgs}
+
+        sudo -u postgres "$NEWBIN/pg_upgrade" \
+          --old-datadir "$OLDDATA" --new-datadir "$NEWDATA" \
+          --old-bindir "$OLDBIN" --new-bindir "$NEWBIN" \
+          "$@"
+      ''
+    )
+  ];
 }