refactor: rename vps host -> neptun (solar-system theme)

git-mv hosts/vps->neptun, secrets/vps.yaml->neptun.yaml; update flake, .sops.yaml
rules, hostName, sops paths, README/CLAUDE. Off-repo host key dir renamed too.

hosts/neptun/secrets.nix

@@ -0,0 +1,16 @@
+{ config, ... }:
+
+# sops-nix wiring for neptun (netcup VPS). Encrypted values in ../../secrets/neptun.yaml,
+# decrypted with the VPS's own SSH host key (recipient in ../../.sops.yaml).
+# The host key is pre-generated on the laptop and shipped at install
+# (nixos-anywhere --extra-files -> /etc/ssh/ssh_host_ed25519_key).
+{
+  sops.defaultSopsFile = ../../secrets/neptun.yaml;
+  sops.age.sshKeyPaths = [ "/etc/ssh/ssh_host_ed25519_key" ];
+
+  sops.secrets.tailscale_authkey = { };
+
+  # darman's console password (own hash = distinct from jupiter/mercury).
+  sops.secrets.darman_password.neededForUsers = true;
+  users.users.darman.hashedPasswordFile = config.sops.secrets.darman_password.path;
+}