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/tests/.pi-map.index.md

@@ -0,0 +1,39 @@
+# backend/tests (index)
+dir: backend/tests
+
+## role
+Test suite providing unit and integration coverage for the backend's API endpoints, clients, configuration, utilities, and services.
+## parent
+index: backend/.pi-map.index.md
+map: backend/.pi-map.md
+## children
+-
+## files
+- __init__.py
+- test_api.py
+- test_backups.py
+- test_config.py
+- test_domain_media.py
+- test_jellyseerr_client.py
+- test_jobs.py
+- test_mailer.py
+- test_media_index.py
+- test_observability.py
+- test_path_utils.py
+- test_services.py
+- test_ssh_client.py
+- test_targets.py
+- test_utils.py
+- test_widgets.py
+## links
+index: backend/tests/.pi-map.index.md
+map: backend/tests/.pi-map.md
+## workflows
+- change tests behavior
+  read: __init__.py, test_api.py, test_backups.py
+- change tests CLI
+  read: test_jellyseerr_client.py, test_ssh_client.py
+- change tests config
+  read: test_config.py
+## dirty
+-