diff --git a/backend/src/main/resources/db/migration/V42__add_queued_per_person_unique_index.sql b/backend/src/main/resources/db/migration/V42__add_queued_per_person_unique_index.sql
new file mode 100644
index 00000000..5e4194bd
--- /dev/null
+++ b/backend/src/main/resources/db/migration/V42__add_queued_per_person_unique_index.sql
@@ -0,0 +1,3 @@
+CREATE UNIQUE INDEX idx_training_runs_queued_per_person
+    ON ocr_training_runs(person_id)
+    WHERE status = 'QUEUED';
diff --git a/backend/src/test/java/org/raddatz/familienarchiv/service/SenderModelServiceTest.java b/backend/src/test/java/org/raddatz/familienarchiv/service/SenderModelServiceTest.java
index be41de41..57318758 100644
--- a/backend/src/test/java/org/raddatz/familienarchiv/service/SenderModelServiceTest.java
+++ b/backend/src/test/java/org/raddatz/familienarchiv/service/SenderModelServiceTest.java
@@ -2,6 +2,8 @@ package org.raddatz.familienarchiv.service;
 
 import org.junit.jupiter.api.BeforeEach;
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.extension.ExtendWith;
+import org.mockito.junit.jupiter.MockitoExtension;
 import org.raddatz.familienarchiv.model.OcrTrainingRun;
 import org.raddatz.familienarchiv.model.SenderModel;
 import org.raddatz.familienarchiv.model.TrainingStatus;
@@ -19,6 +21,7 @@ import static org.assertj.core.api.Assertions.assertThat;
 import static org.mockito.ArgumentMatchers.*;
 import static org.mockito.Mockito.*;
 
+@ExtendWith(MockitoExtension.class)
 class SenderModelServiceTest {
 
     SenderModelRepository senderModelRepository;
@@ -40,8 +43,9 @@ class SenderModelServiceTest {
         txTemplate = mock(TransactionTemplate.class);
         trainingDataExportService = mock(TrainingDataExportService.class);
 
-        // Execute transaction callbacks inline so unit tests run without a real DataSource
-        when(txTemplate.execute(any())).thenAnswer(inv -> {
+        // Execute transaction callbacks inline so unit tests run without a real DataSource.
+        // lenient: not every test hits the txTemplate path, but the setup is shared.
+        lenient().when(txTemplate.execute(any())).thenAnswer(inv -> {
             TransactionCallback<?> callback = inv.getArgument(0);
             return callback.doInTransaction(null);
         });
@@ -265,7 +269,6 @@ class SenderModelServiceTest {
                 .modelName("sender_" + personId).build();
         when(trainingRunRepository.findFirstByPersonIdAndStatus(personId, TrainingStatus.RUNNING))
                 .thenReturn(Optional.of(runningRun));
-        when(blockRepository.countManualKurrentBlocksByPerson(personId)).thenReturn(10L);
         when(trainingRunRepository.save(any())).thenAnswer(inv -> {
             OcrTrainingRun r = inv.getArgument(0);
             if (r.getId() == null) r.setId(UUID.randomUUID());