test(dashboard): add empty-queue guard and boundary tests for contributor cap

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

backend/src/test/java/org/raddatz/familienarchiv/service/TranscriptionQueueServiceTest.java

@@ -53,6 +53,38 @@ class TranscriptionQueueServiceTest {
         assertThat(result.get(0).annotationCount()).isEqualTo(0);
     }
 
+    @Test
+    void getSegmentationQueue_returnsEmptyList_whenQueueIsEmpty() {
+        when(documentRepository.findSegmentationQueue(5)).thenReturn(List.of());
+
+        List<TranscriptionQueueItemDTO> result = service.getSegmentationQueue();
+
+        assertThat(result).isEmpty();
+        verifyNoInteractions(auditLogQueryService);
+    }
+
+    @Test
+    void getSegmentationQueue_returnsAllFive_andHasMoreFalse_whenExactlyFiveContributors() {
+        UUID docId = UUID.randomUUID();
+        TranscriptionQueueProjection proj = mockQueueProjection(docId, "Brief", null, 0, 0, 0);
+        when(documentRepository.findSegmentationQueue(5)).thenReturn(List.of(proj));
+
+        List<ActivityActorDTO> fiveActors = List.of(
+            new ActivityActorDTO("A1", "#111", "Alice One"),
+            new ActivityActorDTO("A2", "#222", "Alice Two"),
+            new ActivityActorDTO("A3", "#333", "Alice Three"),
+            new ActivityActorDTO("A4", "#444", "Alice Four"),
+            new ActivityActorDTO("A5", "#555", "Alice Five")
+        );
+        when(auditLogQueryService.findContributorsPerDocument(List.of(docId)))
+                .thenReturn(Map.of(docId, fiveActors));
+
+        List<TranscriptionQueueItemDTO> result = service.getSegmentationQueue();
+
+        assertThat(result.get(0).contributors()).hasSize(5);
+        assertThat(result.get(0).hasMoreContributors()).isFalse();
+    }
+
     @Test
     void getSegmentationQueue_mapsDocumentDateWhenPresent() {
         LocalDate date = LocalDate.of(1920, 6, 15);