refactor(schedule): extract application/ layer (stage 5)

- Move service.py -> application/service.py (SchedulerService class name
  unchanged; build_object_store / build_storage_http_client move along)
- main.py imports schedule.application.service
- Rewrite worker.py's lazy `from schedule.service import build_object_store`
  and test_worker.py's mock patch string targets — same class of bug as the
  test_janitor patch strings (silent no-op until the old file is deleted)
- Delete flat service.py (orphaned; only docstring refs remain in
  orchestrator, cleaned up in stage 6)
- Zero behavior change; schedule/pyproject.toml untouched

Co-Authored-By: Claude <noreply@anthropic.com>

schedule/tests/test_worker.py

@@ -61,7 +61,7 @@ def test_store_for_custom_bucket_uses_build_factory(monkeypatch: pytest.MonkeyPa
     """A custom bucket_name must produce a store via ``build_object_store``."""
     custom_store = MagicMock(name="custom_store")
     with patch(
-        "schedule.service.build_object_store",
+        "schedule.application.service.build_object_store",
         return_value=custom_store,
     ) as mock_build:
         executor, _, _ = _make_executor()
@@ -75,7 +75,7 @@ def test_store_for_custom_bucket_is_cached(monkeypatch: pytest.MonkeyPatch) -> N
     """Repeated lookups for the same custom bucket must hit the cache."""
     custom_store = MagicMock(name="custom_store")
     with patch(
-        "schedule.service.build_object_store",
+        "schedule.application.service.build_object_store",
         return_value=custom_store,
     ) as mock_build:
         executor, _, _ = _make_executor()