split docker-compose into core and observability stacks; add ntfy, alertmanager, blackbox exporter, and otel-collector services; configure prometheus alerting pipeline

prometheus/alertmanager/alertmanager.yml

@@ -0,0 +1,11 @@
+route:
+  receiver: ntfy
+  group_by: ['alertname']
+  repeat_interval: 8760h
+  group_wait: 10s
+
+receivers:
+  - name: ntfy
+    webhook_configs:
+      - url: http://ntfy:80/alerts?template=alertmanager
+        send_resolved: true