Adds JOURNEY_DOCUMENT_ALREADY_ADDED to ErrorCode, an existsByGeschichteIdAndDocumentId() repo method, and a 409 guard in JourneyItemService.append() — the error code was registered on the frontend but never thrown on the backend, allowing concurrent tabs to add the same document twice. Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>