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/media_library_viewer_api/routers/settings.py

@@ -40,8 +40,6 @@ class MonitoringMachineInput(BaseModel):
     ssh_private_key: str = ""
     ssh_private_key_passphrase: str = ""
     password: str = ""
-    media_root: str = ""
-    path_prefix: str = ""
     notes: str = ""