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/src/.pi-map.index.md

@@ -0,0 +1,20 @@
+# backend/src (index)
+dir: backend/src
+
+## role
+Root source directory of the backend application containing server configuration, business logic, API routes, and data access layers.
+## parent
+index: backend/.pi-map.index.md
+map: backend/.pi-map.md
+## children
+- backend/src/media_library_viewer_api
+  index: backend/src/media_library_viewer_api/.pi-map.index.md
+  map: backend/src/media_library_viewer_api/.pi-map.md
+## files
+## links
+index: backend/src/.pi-map.index.md
+map: backend/src/.pi-map.md
+## workflows
+-
+## dirty
+-