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.

docs/superpowers/plans/.pi-map.md

@@ -0,0 +1,19 @@
+# docs/superpowers/plans
+dir: docs/superpowers/plans
+
+index: docs/superpowers/plans/.pi-map.index.md
+
+## role
+Documentation directory containing detailed implementation plans for specific project features or standalone systems.
+## files
+- 2026-05-11-backup-monitoring.md | Implementation plan for building a standalone backup monitoring system with FastAPI backend, SQLite storage, and React frontend that receives HTTP reports, tracks jobs/runs/alerts, and provides dashboards with validation and alerting. | dep: FastAPI, SQLite, Pydantic, React, TypeScript, TanStack Query, D3, MUI, Python uuid/time/json modules
+## arch
+Flat-file markdown documentation following a date-prefixed naming convention (`YYYY-MM-DD-feature.md`) to organize chronological planning artifacts.
+## tags
+backup, monitoring, fastapi, sqlite, react, 2026, 05, 11
+## symbols
+-
+## workflows
+-
+## dirty
+-