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.

frontend/src/pages/__tests__/Media.test.tsx

@@ -34,11 +34,6 @@ function machineFixture(
 		password_set: false,
 		media_root: "",
 		path_prefix: "",
-		jellyfin_url: "",
-		jellyfin_user_id: "",
-		jellyfin_api_key_set: false,
-		jellyseerr_url: "",
-		jellyseerr_api_key_set: false,
 		notes: "",
 		...overrides,
 	};