refactor: update Nginx configuration and Docker setup

- Change user directive in Nginx configuration from 'nginx' to 'www-data'.
- Update upstream server configurations in Nginx to use 'localhost' instead of service names.
- Modify Nginx server block to redirect HTTP to a status page instead of Grafana.
- Rename Alertmanager location from '/alertmanager/' to '/alerts/' for consistency.
- Remove deprecated status page configuration and related files.
- Adjust Prometheus configuration to reflect the new Docker network settings.

infra/nginx/conf.d/uptime-kuma.conf

@@ -1,12 +1,6 @@
 # Uptime Kuma Nginx Configuration
 # Proxies requests to Uptime Kuma status page
 
-# Upstream for Uptime Kuma
-upstream uptime_kuma_backend {
-    server uptime-kuma:3001;
-    keepalive 32;
-}
-
 # Status page location
 location /status {
     # Rate limiting