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/exporter.yml

@@ -0,0 +1,34 @@
+---
+# Prometheus nginx exporter (optional, gated by nginx_exporter_enabled).
+# Installs nginx-prometheus-exporter and exposes a loopback-only stub_status
+# endpoint that the exporter scrapes.
+
+- name: Install nginx-prometheus-exporter
+  ansible.builtin.package:
+    name: "{{ nginx_exporter_package }}"
+    state: present
+
+- name: Deploy nginx stub_status endpoint
+  ansible.builtin.template:
+    src: stub-status.conf.j2
+    dest: "{{ nginx_conf_dir }}/99-stub-status.conf"
+    owner: root
+    group: root
+    mode: "0644"
+  notify: Reload nginx
+
+- name: Deploy exporter environment file
+  ansible.builtin.template:
+    src: exporter-env.conf.j2
+    dest: "{{ nginx_exporter_args_file }}"
+    owner: root
+    group: root
+    mode: "0644"
+  notify: Restart nginx exporter
+
+- name: Enable and start nginx exporter
+  ansible.builtin.systemd:
+    name: "{{ nginx_exporter_service }}"
+    enabled: true
+    state: started
+    daemon_reload: true