_transition previously called update_campaign then add_event as two
separate commits, so a failed add_event left a status change with no
audit row. Phase 2 launches campaigns that authorise spend, so a
compliance hole like that has to close first.
Add CampaignRepository.update_campaign_with_event, which runs the
UPDATE and the event INSERT on one connection inside one transaction
and commits once. Route campaign transitions (submit/approve/reject)
through it instead of the update_campaign + add_event pair; update_campaign
and add_event themselves are unchanged for other callers.
Also update test_campaign_api.py's FakeRepo/monkeypatch to implement the
new repository method, since production code now calls it in the
submit/approve/reject path.
