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/workers/.pi-map.md

@@ -0,0 +1,28 @@
+# backend/src/media_library_viewer_api/workers
+dir: backend/src/media_library_viewer_api/workers
+
+index: backend/src/media_library_viewer_api/workers/.pi-map.index.md
+
+## role
+Background worker package providing subprocess-based media index building from Jellyfin libraries with progress tracking and cancellation support.
+## files
+- __init__.py | Marks the directory as a Python package for worker entrypoints used in background task processing.
+- media_index_worker.py | Subprocess worker that builds a media index from Jellyfin libraries with progress tracking and cooperative cancellation support. | exp: func:_set_build_metadata(index: MediaIndex, state: dict[str, Any]) → None, call:state.items, call:index.set_metadata, func:_cancel_requested(index: MediaIndex) → bool, call:index.status, func:_start_state(index: MediaIndex, pid: int, library_count: int) → None, call:_set_build_metadata, func:_progress_callback(index: MediaIndex, pid: int, state: dict[str, Any]) → None, call:_set_build_metadata, call:state.get, func:run_build(final_index_path: str | Path, staging_index_path: str | Path) → int, call:get_settings, call:configure_logging, call:logger.info, call:describe_settings, call:get_jellyfin_client, call:get_user_id, call:client.libraries, call:MediaIndex, call:os.getpid, call:time.perf_counter, call:Path, call:staging_path.unlink, call:len, call:_start_state, call:build_media_index, call:_progress_callback, call:_cancel_requested, call:os.replace, call:completed_index.status, call:_set_build_metadata, call:logger.exception, call:str, call:staging_path.exists, func:main() → int, call:argparse.ArgumentParser, call:parser.add_argument, call:parser.parse_args, call:run_build | dep: argparse, logging, os, time, pathlib, typing, media_library_viewer_api.config, media_library_viewer_api.dependencies, media_library_viewer_api.logging_utils, media_library_viewer_api.services.media_index
+## arch
+Subprocess worker pattern with cooperative cancellation, designed to be spawned and monitored by a parent process for long-running media indexing tasks.
+## tags
+call:, metadata, set, index, call:get, media, progress, worker
+## symbols
+- _set_build_metadata
+- _cancel_requested
+- _start_state
+- _progress_callback
+- run_build
+- main
+- call:state.items
+- call:index.set_metadata
+## workflows
+- change workers behavior
+  read: __init__.py, media_index_worker.py
+## dirty
+-