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/services/.pi-map.index.md

@@ -2,7 +2,7 @@
 dir: backend/src/media_library_viewer_api/services
 
 ## role
-Backend service layer providing business logic for media library management, including media indexing, backup monitoring, email notifications, SSH task execution, settings persistence, and security.
+Backend service layer providing business logic for media library management, backup monitoring, email notifications, SSH task execution, encryption, and persistent settings storage.
 ## parent
 index: backend/src/media_library_viewer_api/.pi-map.index.md
 map: backend/src/media_library_viewer_api/.pi-map.md

backend/src/media_library_viewer_api/services/settings_store.py

@@ -17,16 +17,14 @@ from typing import Any
 
 import paramiko
 
-from media_library_viewer_api.config import get_settings
 from media_library_viewer_api.models.widgets import _validate_config_keys
 
 DEFAULT_SETTINGS_PATH = Path(".cache/media_library_viewer/settings.sqlite")
 LOCAL_MACHINE_ID = "local"
-DEFAULT_SERVICES = ["monitoring", "files", "jellyfin"]
+DEFAULT_SERVICES = ["monitoring", "files"]
 
 
 def _default_local_machine() -> dict[str, Any]:
-    settings = get_settings()
     return {
         "id": LOCAL_MACHINE_ID,
         "name": "This machine",
@@ -42,8 +40,6 @@ def _default_local_machine() -> dict[str, Any]:
         "ssh_private_key": "",
         "ssh_private_key_passphrase": "",
         "password": "",
-        "media_root": settings.media_root,
-        "path_prefix": settings.path_prefix,
         "node_exporter_enabled": False,
         "node_exporter_port": 9100,
         "node_exporter_scrape_host": "",
@@ -285,8 +281,6 @@ class SettingsStore:
             "ssh_private_key_set": bool(data.get("ssh_private_key")),
             "ssh_private_key_passphrase_set": bool(data.get("ssh_private_key_passphrase")),
             "password_set": bool(data.get("password")),
-            "media_root": data.get("media_root", ""),
-            "path_prefix": data.get("path_prefix", ""),
             "node_exporter_enabled": bool(data.get("node_exporter_enabled", False)),
             "node_exporter_port": int(data.get("node_exporter_port", 9100) or 9100),
             "node_exporter_scrape_host": data.get("node_exporter_scrape_host", ""),
@@ -334,8 +328,6 @@ class SettingsStore:
         if password in (None, ""):
             password = (current or {}).get("password", "")
         password = str(password or "")
-        media_root = _current_str("media_root")
-        path_prefix = _current_str("path_prefix")
         node_exporter_enabled = bool(
             payload.get("node_exporter_enabled")
             if payload.get("node_exporter_enabled") is not None
@@ -365,8 +357,6 @@ class SettingsStore:
             "ssh_private_key": ssh_private_key,
             "ssh_private_key_passphrase": ssh_private_key_passphrase,
             "password": password,
-            "media_root": media_root,
-            "path_prefix": path_prefix,
             "node_exporter_enabled": node_exporter_enabled,
             "node_exporter_port": node_exporter_port,
             "node_exporter_scrape_host": node_exporter_scrape_host,
@@ -388,8 +378,6 @@ class SettingsStore:
             "ssh_private_key": "",
             "ssh_private_key_passphrase": "",
             "password": "",
-            "media_root": machine["media_root"],
-            "path_prefix": machine["path_prefix"],
             "node_exporter_enabled": machine["node_exporter_enabled"],
             "node_exporter_port": machine["node_exporter_port"],
             "node_exporter_scrape_host": machine["node_exporter_scrape_host"],
@@ -473,8 +461,6 @@ class SettingsStore:
             "ssh_private_key": data.get("ssh_private_key", ""),
             "ssh_private_key_passphrase": data.get("ssh_private_key_passphrase", ""),
             "password": data.get("password", ""),
-            "media_root": data.get("media_root", ""),
-            "path_prefix": data.get("path_prefix", ""),
             "node_exporter_enabled": bool(data.get("node_exporter_enabled", False)),
             "node_exporter_port": int(data.get("node_exporter_port", 9100) or 9100),
             "node_exporter_scrape_host": data.get("node_exporter_scrape_host", ""),
@@ -512,8 +498,6 @@ class SettingsStore:
             "ssh_private_key": machine["ssh_private_key"],
             "ssh_private_key_passphrase": machine["ssh_private_key_passphrase"],
             "password": machine["password"],
-            "media_root": machine["media_root"],
-            "path_prefix": machine["path_prefix"],
             "node_exporter_enabled": machine["node_exporter_enabled"],
             "node_exporter_port": machine["node_exporter_port"],
             "node_exporter_scrape_host": machine["node_exporter_scrape_host"],