add node_exporter host vitals + quickshell HUD

Prometheus node_exporter enabled on every host, plus a quickshell widget
(SUPER+CTRL+V on terra) to view live CPU/mem/disk/net/uptime without a
separate dashboard.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_011FHr5ug9pu8q4XPrRkFnzJ

hosts/neptun/configuration.nix

@@ -9,6 +9,7 @@
     ../../common.nix              # shared base: user / ssh / nix / firewall
     ../../services/network/caddy.nix
     ../../services/vpn/tailscale.nix
+    ../../services/monitoring/node-exporter.nix
     ../../services/identity/authentik.nix
     ../../services/vpn/headscale.nix
     ../../services/vpn/headplane.nix