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/tasks/main.yml

@@ -60,3 +60,7 @@
   delay: 2
   register: ufw_result
   until: ufw_result is succeeded
+
+- name: Deploy Prometheus valkey exporter
+  ansible.builtin.include_tasks: exporter.yml
+  when: valkey_exporter_enabled | bool