Add observability platform Ansible project

Deploys the Grafana stack (Prometheus/Alertmanager/Loki/Tempo/Grafana with
Alloy as collector) and the ELK stack (Filebeat/Logstash/Elasticsearch/
Kibana) via Docker Compose on a target host. External to the application
repo, pre-built by the mentor.

ansible/group_vars/all.yml

@@ -0,0 +1,29 @@
+---
+# Deployment base path on the target host.
+observability_root: /opt/observability
+
+# Change these tags deliberately when upgrading.
+elastic_version: "9.1.3"
+grafana_version: "12.1.1"
+prometheus_version: "v3.5.0"
+alertmanager_version: "v0.28.1"
+loki_version: "3.5.3"
+tempo_version: "2.8.2"
+alloy_version: "v1.10.2"
+node_exporter_version: "v1.9.1"
+
+# Lab-safe defaults. Override with Ansible Vault for real deployments.
+grafana_admin_user: admin
+grafana_admin_password: "change-me-now"
+
+# Optional Alertmanager webhook. Leave empty to install a no-op receiver.
+alertmanager_webhook_url: ""
+
+# Add application /metrics endpoints here. Example:
+# prometheus_extra_scrape_jobs:
+#   - job_name: checkout-api
+#     targets: ["10.20.0.15:8080", "10.20.0.16:8080"]
+prometheus_extra_scrape_jobs: []
+
+# JVM heap for single-node Elasticsearch. Keep Xms and Xmx equal.
+elasticsearch_heap: 1g