feat: add nginx-prometheus-exporter sidecar for web metrics

docker-compose.yaml

@@ -384,6 +384,26 @@ services:
       timeout: 5s
       retries: 3
 
+  # ── Web metrics: nginx-prometheus-exporter ─────────────────────────────────
+  # Scrapes the `web` container's stub_status endpoint and re-exposes it in
+  # Prometheus format on 9113. Prospero (Sao) scrapes this; see
+  # virgo/ansible/pplg/prometheus.yml.j2 → job_name: 'mnemosyne'.
+  # The Django /metrics endpoint (django-prometheus + custom pipeline metrics
+  # in mcp_server/metrics.py and library/metrics.py) is reached separately
+  # via nginx at /metrics — no sidecar needed for that.
+  web-metrics:
+    image: nginx/nginx-prometheus-exporter:latest
+    command:
+      - --nginx.scrape-uri
+      - http://web:80/nginx_status
+    depends_on:
+      web:
+        condition: service_started
+    ports:
+      - "23191:9113"
+    restart: unless-stopped
+    logging: *default-logging
+
 volumes:
   # Static files written by collectstatic (run by the init sidecar on every
   # `up`). Docker-managed volume — no host path needed; storage is minimal