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/.pi-map.md

@@ -0,0 +1,21 @@
+# backend
+dir: backend
+
+index: backend/.pi-map.index.md
+
+## role
+FastAPI backend service providing REST API endpoints for Jellyfin media browsing, SSH file inspection, and server monitoring.
+## files
+- Dockerfile | Builds a Docker container for a Python 3.11 backend API service using uvicorn | dep: python:3.11-slim, pip, uvicorn, pyproject.toml-based package
+- README.md | Documentation describing the setup, configuration, Docker deployment, and API endpoints of a FastAPI backend for Jellyfin media browsing, SSH file inspection, and server monitoring. | dep: FastAPI, uvicorn, pydantic-settings, Jellyfin, Jellyseerr, SSH, Docker Compose, Alertmanager, Prometheus, Grafana, Authentik/OIDC
+- pyproject.toml | Project configuration file defining dependencies, build system, linting, and testing settings for a FastAPI media library viewer backend. | dep: FastAPI, uvicorn, pydantic-settings, paramiko, requests, python-dotenv, pandas, PyJWT, prometheus-client, python-json-logger, cryptography, hatchling, ruff, pytest, httpx
+## arch
+Containerized Python 3.11 service using FastAPI framework with uvicorn ASGI server, following standard pyproject.toml-based project configuration with defined linting and testing pipelines.
+## tags
+uvicorn, fastapi, python, backend, pyproject, settings, docker, api
+## symbols
+-
+## workflows
+-
+## dirty
+-