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/media_library_viewer_api/routers/.pi-map.index.md

@@ -0,0 +1,32 @@
+# backend/src/media_library_viewer_api/routers (index)
+dir: backend/src/media_library_viewer_api/routers
+
+## role
+FastAPI router package that exposes all REST API endpoints for the media library viewer backend, organized by domain (backups, dashboard, files, jobs, media, monitoring, services, settings, tasks, users, widgets).
+## parent
+index: backend/src/media_library_viewer_api/.pi-map.index.md
+map: backend/src/media_library_viewer_api/.pi-map.md
+## children
+-
+## files
+- __init__.py
+- backups.py
+- dashboard.py
+- files.py
+- jobs.py
+- media.py
+- monitoring.py
+- services.py
+- settings.py
+- tasks.py
+- users.py
+- users_impl.py
+- widgets.py
+## links
+index: backend/src/media_library_viewer_api/routers/.pi-map.index.md
+map: backend/src/media_library_viewer_api/routers/.pi-map.md
+## workflows
+- change routers behavior
+  read: __init__.py, backups.py, dashboard.py
+## dirty
+-