refactor(schedule): extract scheduling/ layer (scheduler + orchestrator)

Stage 3 of the layered refactor. Relocate the two scheduling components
into their own package so that domain / application / scheduling /
execution / infrastructure boundaries actually exist on disk.

- Add schedule/src/schedule/scheduling/__init__.py
- Move scheduler.py (232 lines) -> scheduling/scheduler.py
  (byte-identical via diff; CronScheduler class name unchanged)
- Move orchestrator.py (946 lines) -> scheduling/orchestrator.py
  (byte-identical via diff; DispatchOrchestrator + event constants
  unchanged; NOT further split this round, per plan)
- service.py lines 39-40: import paths rewritten to the new module
- tests/test_janitor.py: rewrite the import + 5 patch() string targets

  The 5 patch() targets ("schedule.orchestrator.session_scope" x3,
  "schedule.orchestrator.asyncio.sleep" x2) were NOT caught by the
  import-line grep — they patch module attributes at runtime and would
  have become dead no-ops after the move (and would hard-raise once
  the old module is deleted in stage 6). Rewriting them to
  "schedule.scheduling.orchestrator.*" keeps the janitor tests meaningfully
  exercising the new module.

- old flat scheduler.py / orchestrator.py left on disk; stage 6 deletes
  them once all layers are extracted.

Validation:
- uv run --package schedule pytest schedule/tests -q: 18 passed
- uv run python -m compileall schedule/src: zero errors
- grep 'from schedule.(scheduler|orchestrator)\\b' (old paths): 0 matches
- grep '"schedule.orchestrator.' (old patch targets): 0 matches
- main.py / worker.py / domain/ / infrastructure/ / pyproject.toml
  byte-identical to HEAD

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

schedule/tests/test_janitor.py

@@ -23,7 +23,7 @@ import pytest
 
 from common.db.models import ScheduleNodeRuns
 from common.eventing import utcnow
-from schedule.orchestrator import NODE_FINISHED_EVENT, DispatchOrchestrator
+from schedule.scheduling.orchestrator import NODE_FINISHED_EVENT, DispatchOrchestrator
 
 
 def _make_orchestrator() -> DispatchOrchestrator:
@@ -195,7 +195,7 @@ async def test_reap_kills_running_row_past_deadline() -> None:
     fake_session.add = MagicMock()
 
     with patch(
-        "schedule.orchestrator.session_scope",
+        "schedule.scheduling.orchestrator.session_scope",
         return_value=_open_session_scope(fake_session),
     ):
         killed = await orch._reap_stuck_node_runs()
@@ -223,7 +223,7 @@ async def test_reap_skips_healthy_row() -> None:
     fake_session.add = MagicMock()
 
     with patch(
-        "schedule.orchestrator.session_scope",
+        "schedule.scheduling.orchestrator.session_scope",
         return_value=_open_session_scope(fake_session),
     ):
         killed = await orch._reap_stuck_node_runs()
@@ -266,7 +266,7 @@ async def test_reap_processes_multiple_rows_in_one_pass() -> None:
     fake_session.add = MagicMock()
 
     with patch(
-        "schedule.orchestrator.session_scope",
+        "schedule.scheduling.orchestrator.session_scope",
         return_value=_open_session_scope(fake_session),
     ):
         killed = await orch._reap_stuck_node_runs()
@@ -289,7 +289,7 @@ async def test_janitor_loop_propagates_cancellation() -> None:
         raise asyncio.CancelledError
 
     with patch(
-        "schedule.orchestrator.asyncio.sleep",
+        "schedule.scheduling.orchestrator.asyncio.sleep",
         side_effect=cancel_on_sleep,
     ):
         with pytest.raises(asyncio.CancelledError):
@@ -321,7 +321,7 @@ async def test_janitor_loop_continues_after_reap_exception() -> None:
             raise asyncio.CancelledError
 
     with patch(
-        "schedule.orchestrator.asyncio.sleep",
+        "schedule.scheduling.orchestrator.asyncio.sleep",
         side_effect=_count_sleeps,
     ):
         with pytest.raises(asyncio.CancelledError):