Fix nginx-exporter port and add alertmanager scrape target

- nginx stub_status moved to port 8888 (port 80 was being caught
  by other server blocks and returning 301)
- nginx-exporter updated to scrape :8888
- Added alertmanager scrape job to Prometheus config (was missing,
  so alertmanager dashboard had no data)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

monitoring/prometheus.yml

@@ -34,6 +34,11 @@ scrape_configs:
         labels:
           instance: prod
 
+  # ── Alertmanager ────────────────────────────────────────────────────
+  - job_name: alertmanager
+    static_configs:
+      - targets: ["alertmanager:9093"]
+
   # ── nginx ──────────────────────────────────────────────────────────
   - job_name: nginx
     static_configs: