fix(journeyitem): use specific error codes in append() — JOURNEY_AT_CAPACITY and GESCHICHTE_TYPE_MISMATCH

- JourneyItemService.append(): replace VALIDATION_ERROR with GESCHICHTE_TYPE_MISMATCH (409 conflict)
  for non-JOURNEY type guard and JOURNEY_AT_CAPACITY (409 conflict) for 100-item cap
- JourneyItemServiceTest: update assertions to expect the new specific error codes
- CLAUDE.md: expand geschichte/ package table entry with GeschichteQueryService and journeyitem/ sub-domain

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

CLAUDE.md

@@ -86,7 +86,8 @@ backend/src/main/java/org/raddatz/familienarchiv/
 │   └── transcription/   TranscriptionBlock, TranscriptionService, TranscriptionBlockQueryService
 ├── exception/           DomainException, ErrorCode, GlobalExceptionHandler
 ├── filestorage/         FileService (S3/MinIO)
-├── geschichte/          Geschichte (story) domain
+├── geschichte/          Geschichte (story) domain — GeschichteService, GeschichteQueryService
+│   └── journeyitem/     JourneyItem sub-domain — JourneyItemService, JourneyItemController
 ├── importing/           CanonicalImportOrchestrator + four loaders (TagTree/PersonRegister/PersonTree/Document) + CanonicalSheetReader
 ├── notification/        Notification domain + SseEmitterRegistry
 ├── ocr/                 OCR domain — OcrService, OcrBatchService, training