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/test/java/org/raddatz/familienarchiv/document/transcription/TranscriptionBlockQueryServiceTest.java

@@ -0,0 +1,35 @@
+package org.raddatz.familienarchiv.document.transcription;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.InjectMocks;
+import org.mockito.Mock;
+import org.mockito.junit.jupiter.MockitoExtension;
+
+import java.util.UUID;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.Mockito.when;
+
+@ExtendWith(MockitoExtension.class)
+class TranscriptionBlockQueryServiceTest {
+
+    @Mock TranscriptionBlockRepository blockRepository;
+    @InjectMocks TranscriptionBlockQueryService queryService;
+
+    @Test
+    void hasBlocks_returns_true_when_a_block_exists() {
+        UUID documentId = UUID.randomUUID();
+        when(blockRepository.existsByDocumentId(documentId)).thenReturn(true);
+
+        assertThat(queryService.hasBlocks(documentId)).isTrue();
+    }
+
+    @Test
+    void hasBlocks_returns_false_when_no_block_exists() {
+        UUID documentId = UUID.randomUUID();
+        when(blockRepository.existsByDocumentId(documentId)).thenReturn(false);
+
+        assertThat(queryService.hasBlocks(documentId)).isFalse();
+    }
+}