Three root causes confirmed via live server investigation (issue #604):
1. ManagementWebSecurityAutoConfiguration applied HTTP Basic auth to the
management port (8081), causing Prometheus to receive 401 HTML responses
instead of metrics. Excluded the auto-config — the Docker network
(archiv-net) provides the security boundary for this internal port.
2. promtail-config.yml had no `job` relabel rule. Grafana's Loki dashboards
query {job="$app"} which matched nothing; logs were in Loki under
compose_service but invisible to every dashboard panel.
3. prometheus.yml had a stale comment claiming the spring-boot target would
be DOWN until micrometer-registry-prometheus was added — it has been
present in pom.xml for some time.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
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