fix(ocr): replace IllegalStateException with DomainException in triggerSenderTraining

Consistent with triggerManualSenderTraining — both defensive paths now use
DomainException.internal(OCR_TRAINING_CONFLICT) when the expected RUNNING row
is not found after creation.

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

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

@@ -297,6 +297,16 @@ class SenderModelServiceTest {
         verify(senderModelRepository, never()).save(any());
     }
 
+    @Test
+    void triggerSenderTraining_throwsDomainException_whenRunningRowMissingAfterDispatch() {
+        when(trainingRunRepository.findFirstByPersonIdAndStatus(personId, TrainingStatus.RUNNING))
+                .thenReturn(Optional.empty());
+
+        org.assertj.core.api.Assertions.assertThatThrownBy(
+                () -> service.triggerSenderTraining(personId, 0))
+                .isInstanceOf(DomainException.class);
+    }
+
     // ─── triggerSenderTraining — queue promotion ──────────────────────────────
 
     // ─── triggerManualSenderTraining ──────────────────────────────────────────