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.

backend/.pi-map.index.md

@@ -0,0 +1,32 @@
+# backend (index)
+dir: backend
+
+## role
+FastAPI backend service providing REST API endpoints for Jellyfin media browsing, SSH file inspection, and server monitoring.
+## parent
+index: ./.pi-map.index.md
+map: ./.pi-map.md
+## children
+- backend/.pytest_cache
+  index: backend/.pytest_cache/.pi-map.index.md
+  map: backend/.pytest_cache/.pi-map.md
+- backend/.ruff_cache
+  index: backend/.ruff_cache/.pi-map.index.md
+  map: backend/.ruff_cache/.pi-map.md
+- backend/src
+  index: backend/src/.pi-map.index.md
+  map: backend/src/.pi-map.md
+- backend/tests
+  index: backend/tests/.pi-map.index.md
+  map: backend/tests/.pi-map.md
+## files
+- Dockerfile
+- README.md
+- pyproject.toml
+## links
+index: backend/.pi-map.index.md
+map: backend/.pi-map.md
+## workflows
+-
+## dirty
+-