feat(monitoring): add nginx, postgres, valkey prometheus exporters

- nginx: nginx-prometheus-exporter gated by nginx_exporter_enabled,
  deploys loopback-only stub_status (8090), exporter on 9113
- postgres: postgres_exporter gated by postgres_exporter_enabled,
  creates postgres_exporter role with pg_monitor, exporter on 9187
- valkey: redis_exporter gated by valkey_exporter_enabled, adds
  read-only ACL user to users.acl, exporter on 9121

Each exporter follows the nut pattern (tasks/exporter.yml included from
main.yml, OS vars, 0600 secret env files, password asserts).

roles/valkey/handlers/main.yml

@@ -13,6 +13,11 @@
     name: "{{ valkey_service_name }}"
     state: reloaded
 
+- name: Restart valkey exporter
+  ansible.builtin.systemd:
+    name: "{{ valkey_exporter_service }}"
+    state: restarted
+
 - name: Update GRUB
   ansible.builtin.shell: |
     if command -v update-grub &> /dev/null; then