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/nginx/tasks/main.yml

@@ -207,3 +207,7 @@
     name: nginx
     enabled: true
     state: started
+
+- name: Deploy Prometheus nginx exporter
+  ansible.builtin.include_tasks: exporter.yml
+  when: nginx_exporter_enabled | bool