fix(obs): wire Prometheus metrics and Loki job label for Grafana dashboards

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>

infra/observability/promtail/promtail-config.yml

@@ -28,3 +28,5 @@ scrape_configs:
         target_label: 'compose_project'
       - source_labels: ['__meta_docker_container_log_stream']
         target_label: 'logstream'
+      - source_labels: ['__meta_docker_container_label_com_docker_compose_service']
+        target_label: 'job'