chore: archive observability-service-registry, track pi-map artifacts

- Archive the completed observability-service-registry SDD change into
  openspec/changes/archive/ (delivered across 5 slices; only
  jellyfin-service-registry remains active).
- Stop ignoring .pi-map.md / .pi-map.index.md so the navigation maps are
  versioned alongside the code, and add the regenerated map pairs repo-wide.

monitoring/prometheus/rules/.pi-map.index.md

@@ -0,0 +1,19 @@
+# monitoring/prometheus/rules (index)
+dir: monitoring/prometheus/rules
+
+## role
+Defines Prometheus alerting rules for monitoring backup job failures and observability stack health.
+## parent
+index: monitoring/prometheus/.pi-map.index.md
+map: monitoring/prometheus/.pi-map.md
+## children
+-
+## files
+- backup_alerts.yml
+## links
+index: monitoring/prometheus/rules/.pi-map.index.md
+map: monitoring/prometheus/rules/.pi-map.md
+## workflows
+-
+## dirty
+-

monitoring/prometheus/rules/.pi-map.md

@@ -0,0 +1,19 @@
+# monitoring/prometheus/rules
+dir: monitoring/prometheus/rules
+
+index: monitoring/prometheus/rules/.pi-map.index.md
+
+## role
+Defines Prometheus alerting rules for monitoring backup job failures and observability stack health.
+## files
+- backup_alerts.yml | Defines Prometheus alerting rules for backup job failures and observability stack health monitoring. | dep: Prometheus, Alertmanager, Grafana, manage_backup_runs_total metric, manage_backup_runs_last_success_timestamp metric, up metric
+## arch
+Configuration-as-code pattern using declarative YAML rule files consumed directly by the Prometheus alerting engine.
+## tags
+backup, prometheus, alerts, defines, alerting, rules, job, failures
+## symbols
+-
+## workflows
+-
+## dirty
+-