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/templates/stub-status.conf.j2

@@ -0,0 +1,12 @@
+# Managed by Ansible - DO NOT EDIT MANUALLY
+# stub_status endpoint for nginx-prometheus-exporter (loopback only)
+server {
+    listen {{ nginx_exporter_stub_status_listen }};
+    server_name localhost;
+
+    location /stub_status {
+        stub_status;
+        allow 127.0.0.1;
+        deny all;
+    }
+}