refactor(settings): drop jellyfin machine service + dead machine path fields

Slice 1 of jellyfin-service-registry. Jellyfin is configured exclusively
via the service registry now; the machine-level media_root/path_prefix
fields were dead duplicates of the global config.

- services/settings_store.py: DEFAULT_SERVICES no longer includes
  "jellyfin" (now ["monitoring", "files"]). Removed machine-level
  media_root/path_prefix from _default_local_machine, _row_to_machine,
  _normalize_machine_payload, _seed_local_machine, get_machine_config,
  and upsert_machine. _default_local_machine no longer reads global
  config, so the get_settings import is dropped.
- routers/settings.py: removed media_root/path_prefix from
  MonitoringMachineInput (dead API input; store already ignored them).

The global config remote_media_root/path_prefix properties + path_utils.py
are unchanged (files.py and media_index still use them for Jellyfin->SSH
path resolution). ruff clean; 239 backend tests pass.

backend/src/.pi-map.index.md

@@ -2,7 +2,7 @@
 dir: backend/src
 
 ## role
-Root source directory of the backend application containing server configuration, business logic, API routes, and data access layers.
+Root source directory serving as the entry point for the backend application.
 ## parent
 index: backend/.pi-map.index.md
 map: backend/.pi-map.md