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/integrations/.pi-map.index.md

@@ -0,0 +1,29 @@
+# backend/src/media_library_viewer_api/integrations (index)
+dir: backend/src/media_library_viewer_api/integrations
+
+## role
+Defines and registers third-party service integrations (e.g., Alertmanager, Grafana, Jellyfin, Prometheus) with typed configs, secrets, and widget definitions for the media library viewer API.
+## 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
+- alertmanager.py
+- base.py
+- grafana.py
+- jellyfin.py
+- jellyseerr.py
+- nextcloud.py
+- prometheus.py
+- registry.py
+- ssh_tasks.py
+## links
+index: backend/src/media_library_viewer_api/integrations/.pi-map.index.md
+map: backend/src/media_library_viewer_api/integrations/.pi-map.md
+## workflows
+- change integrations behavior
+  read: __init__.py, alertmanager.py, base.py
+## dirty
+-