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/services/.pi-map.index.md

@@ -0,0 +1,33 @@
+# backend/src/media_library_viewer_api/services (index)
+dir: backend/src/media_library_viewer_api/services
+
+## role
+Backend service layer providing business logic for media library management, including media indexing, backup monitoring, email notifications, SSH task execution, settings persistence, and security.
+## 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
+- backup_alert_engine.py
+- backup_poller.py
+- db_maintenance.py
+- known_hosts.py
+- mail_queue.py
+- mailer.py
+- mailer_impl.py
+- media_index.py
+- media_index_impl.py
+- secrets.py
+- settings_store.py
+- targets.py
+- task_runner.py
+## links
+index: backend/src/media_library_viewer_api/services/.pi-map.index.md
+map: backend/src/media_library_viewer_api/services/.pi-map.md
+## workflows
+- change services behavior
+  read: __init__.py, backup_alert_engine.py, backup_poller.py
+## dirty
+-