fix(backend): store error rate for segmentation training runs

setCer() was called for recognition training but not for segmentation.
The OCR service now returns cer = 1 - accuracy for segtrain; persist it
so the admin panel can display Fehlerrate for both training types.

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

backend/src/main/java/org/raddatz/familienarchiv/service/OcrTrainingService.java

@@ -162,11 +162,12 @@ public class OcrTrainingService {
             return Objects.requireNonNull(txTemplate.execute(status -> {
                 run.setStatus(TrainingStatus.DONE);
                 run.setCompletedAt(Instant.now());
+                run.setCer(result.cer());
                 run.setLoss(result.loss());
                 run.setAccuracy(result.accuracy());
                 run.setEpochs(result.epochs());
                 OcrTrainingRun updated = trainingRunRepository.save(run);
-                log.info("[trainingRun={}] Segmentation training completed — epochs={}", runId, result.epochs());
+                log.info("[trainingRun={}] Segmentation training completed — cer={} epochs={}", runId, result.cer(), result.epochs());
                 return updated;
             }));
         } catch (Exception e) {