feat: add lead-only status change rule for boards and update related logic

backend/tests/test_board_schema.py

@@ -87,15 +87,18 @@ def test_board_rule_toggles_have_expected_defaults() -> None:
     assert created.require_approval_for_done is True
     assert created.require_review_before_done is False
     assert created.block_status_changes_with_pending_approval is False
+    assert created.only_lead_can_change_status is False
 
     updated = BoardUpdate(
         require_approval_for_done=False,
         require_review_before_done=True,
         block_status_changes_with_pending_approval=True,
+        only_lead_can_change_status=True,
     )
     assert updated.require_approval_for_done is False
     assert updated.require_review_before_done is True
     assert updated.block_status_changes_with_pending_approval is True
+    assert updated.only_lead_can_change_status is True
 
 
 def test_onboarding_confirm_requires_goal_fields() -> None: