feat(monitoring): add Portal-1 and Vultr dashboards

- scrape Portal-1 node metrics over Tailscale
- add a hardened Vultr billing and bandwidth exporter
- provision host health and infrastructure cost dashboards

systems/portal-1/default.nix

@@ -12,6 +12,7 @@
     "${inputs.self}/common/optional/tailscale.nix"
     ./disk-config.nix
     ./haproxy
+    ./monitoring.nix
   ];
 
   nixpkgs.hostPlatform = "x86_64-linux";