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

blackbox/blackbox.yml

@@ -0,0 +1,10 @@
+modules:
+  http_2xx:
+    prober: http
+    timeout: 5s
+    http:
+      valid_status_codes: []
+      method: GET
+      preferred_ip_protocol: "ip4"
+      tls_config:
+        insecure_skip_verify: true