secrets: migrate all hosts from sops to OpenBao vault

Replace sops-nix secrets with OpenBao vault secrets across all hosts.
Hardcode root password hash, add extractKey option to vault-secrets
module, update Terraform with secrets/policies for all hosts, and
create AppRole provisioning playbook.

Hosts migrated: ha1, monitoring01, ns1, ns2, http-proxy, nix-cache01
Wave 1 hosts (nats1, jelly01, pgdb1) get AppRole policies only.

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

services/monitoring/alerttonotify.nix

@@ -1,12 +1,18 @@
 { pkgs, config, ... }:
 {
-  sops.secrets."nats_nkey" = { };
+  vault.secrets.nats-nkey = {
+    secretPath = "shared/nats/nkey";
+    extractKey = "nkey";
+    outputDir = "/run/secrets/nats_nkey";
+    services = [ "alerttonotify" ];
+  };
+
   systemd.services."alerttonotify" = {
     enable = true;
     wants = [ "network-online.target" ];
     after = [
       "network-online.target"
-      "sops-nix.service"
+      "vault-secret-nats-nkey.service"
     ];
     wantedBy = [ "multi-user.target" ];
     restartIfChanged = true;