terraform: add nixos-exporter shared policy

- Create shared policy granting all hosts access to nixos-exporter nkey
- Add policy to both manual and generated host AppRoles
- Remove duplicate kanidm01/monitoring02 entries from hosts-generated.tf

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

terraform/vault/approle.tf

@@ -15,6 +15,17 @@ path "secret/data/shared/homelab-deploy/*" {
 EOT
 }
 
+# Shared policy for nixos-exporter NATS cache sharing
+resource "vault_policy" "nixos_exporter" {
+  name = "nixos-exporter"
+
+  policy = <<EOT
+path "secret/data/shared/nixos-exporter/*" {
+  capabilities = ["read", "list"]
+}
+EOT
+}
+
 # Define host access policies
 locals {
   host_policies = {
@@ -131,7 +142,7 @@ resource "vault_approle_auth_backend_role" "hosts" {
   backend   = vault_auth_backend.approle.path
   role_name = each.key
   token_policies = concat(
-    ["${each.key}-policy", "homelab-deploy"],
+    ["${each.key}-policy", "homelab-deploy", "nixos-exporter"],
     lookup(each.value, "extra_policies", [])
   )