fix(annotations): cascade-delete transcription block when annotation is deleted

The DELETE endpoint was returning 500 due to a FK constraint violation.
`deleteAnnotation` now calls `blockRepository.deleteByAnnotationId()`
before removing the annotation.

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

backend/src/main/java/org/raddatz/familienarchiv/repository/TranscriptionBlockRepository.java

@@ -16,6 +16,8 @@ public interface TranscriptionBlockRepository extends JpaRepository<Transcriptio
 
     Optional<TranscriptionBlock> findByAnnotationId(UUID annotationId);
 
+    void deleteByAnnotationId(UUID annotationId);
+
     int countByDocumentId(UUID documentId);
 
     @Query("""

backend/src/main/java/org/raddatz/familienarchiv/service/AnnotationService.java

@@ -6,6 +6,7 @@ import org.raddatz.familienarchiv.exception.DomainException;
 import org.raddatz.familienarchiv.exception.ErrorCode;
 import org.raddatz.familienarchiv.model.DocumentAnnotation;
 import org.raddatz.familienarchiv.repository.AnnotationRepository;
+import org.raddatz.familienarchiv.repository.TranscriptionBlockRepository;
 import org.springframework.stereotype.Service;
 import org.springframework.transaction.annotation.Transactional;
 
@@ -17,6 +18,7 @@ import java.util.UUID;
 public class AnnotationService {
 
     private final AnnotationRepository annotationRepository;
+    private final TranscriptionBlockRepository blockRepository;
 
     public List<DocumentAnnotation> listAnnotations(UUID documentId) {
         return annotationRepository.findByDocumentId(documentId);
@@ -70,6 +72,7 @@ public class AnnotationService {
             throw DomainException.forbidden("Only the annotation author can delete it");
         }
 
+        blockRepository.deleteByAnnotationId(annotationId);
         annotationRepository.delete(annotation);
     }