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>

docker-compose.yml

@@ -372,7 +372,7 @@ services:
   nginx-exporter:
     image: nginx/nginx-prometheus-exporter:latest
     command:
-      - -nginx.scrape-uri=http://host.docker.internal:80/nginx_status
+      - -nginx.scrape-uri=http://host.docker.internal:8888/nginx_status
     extra_hosts:
       - "host.docker.internal:host-gateway"
     restart: unless-stopped