Adds JOURNEY_ITEM_ADDED, JOURNEY_ITEM_REMOVED, JOURNEY_ITEMS_REORDERED (last is ROLLUP_ELIGIBLE — drag-heavy editing produces many events). Adds JOURNEY_ITEM_NOT_FOUND (404) and JOURNEY_ITEM_POSITION_CONFLICT (409) to ErrorCode for IDOR protection and concurrent-edit feedback. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>