chore: remove dead machine-level Jellyfin/Jellyseerr fields

Follow-up #1 to the service-registry change. Jellyfin/Jellyseerr now resolve
from the service registry, so the machine-level app fields are dead config.

- dependencies.py: drop dead _jellyseerr_client_for; simplify _resolve_machine
  to SSH-only.
- settings_store.py + routers/settings.py: remove jellyfin_*/jellyseerr_* from
  machine default config, get_machine_config, normalization, row mappers, and
  MachineInput.
- frontend types + Settings.tsx: drop the fields and the Jellyfin/Jellyseerr
  form sections + service options.
- Update frontend test fixtures.

Existing DB rows may still carry these keys in config_json; they are inert and
drop on the next machine save. Verification: backend ruff clean, pytest 222;
frontend lint 0 errors, build success, 70 tests.

backend/src/media_library_viewer_api/routers/settings.py

@@ -43,11 +43,6 @@ class MonitoringMachineInput(BaseModel):
     password: str = ""
     media_root: str = ""
     path_prefix: str = ""
-    jellyfin_url: str = ""
-    jellyfin_user_id: str = ""
-    jellyfin_api_key: str = ""
-    jellyseerr_url: str = ""
-    jellyseerr_api_key: str = ""
     notes: str = ""