feat(transcription): expose hasBlocks on TranscriptionBlockQueryService (#697)

Domain-service wrapper over existsByDocumentId so other domains can ask
"does this document have any transcription blocks?" without reaching into
the repository.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/main/java/org/raddatz/familienarchiv/document/transcription/TranscriptionBlockQueryService.java

@@ -17,6 +17,10 @@ public class TranscriptionBlockQueryService {
 
     private final TranscriptionBlockRepository blockRepository;
 
+    public boolean hasBlocks(UUID documentId) {
+        return blockRepository.existsByDocumentId(documentId);
+    }
+
     public Map<UUID, Integer> getCompletionStats(List<UUID> documentIds) {
         if (documentIds.isEmpty()) return Map.of();
         Map<UUID, Integer> result = new HashMap<>();