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/templates/users.acl.j2

@@ -2,3 +2,6 @@ user default on >{{ valkey_admin_password }} ~* &* +@all
 {% for acl_user in valkey_acl_users %}
 user {{ acl_user.username }} on >{{ acl_user.password }} {% for pattern in acl_user.keypattern.split() %}~{{ pattern }} {% endfor %}{{ acl_user.commands | default('+@all -@dangerous') }}
 {% endfor %}
+{% if valkey_exporter_enabled | bool %}
+user {{ valkey_exporter_user }} on >{{ valkey_exporter_password }} ~* {{ valkey_exporter_commands }}
+{% endif %}