feat(training): add segmentation training pipeline and complete Part 6

- Add /segtrain endpoint to OCR service (ZIP upload, ketos.segtrain,
  backup rotation, in-process model reload)
- Add segtrainModel() to OcrClient and RestClientOcrClient (10-min timeout,
  X-Training-Token header)
- Add SegmentationTrainingExportService: PAGE XML export with polygon
  de-normalization and per-page PNG rendering via PDFBox
- Add GET /api/ocr/segmentation-training-data/export endpoint
- Make TranscriptionBlock.text nullable for segmentation-only blocks
  (V31 migration)
- Add Paraglide i18n translation keys for all training UI strings (de/en/es)
- Pass source prop from TranscriptionEditView to TranscriptionBlock

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

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

@@ -20,6 +20,7 @@ class OcrClientDefaultStreamTest {
                     new OcrBlockResult(1, 0.1, 0.1, 0.8, 0.04, null, "Line 3"));
             }
             @Override public TrainingResult trainModel(byte[] zip) { return null; }
+            @Override public TrainingResult segtrainModel(byte[] zip) { return null; }
         };
 
         List<OcrStreamEvent> events = new ArrayList<>();
@@ -50,6 +51,7 @@ class OcrClientDefaultStreamTest {
         OcrClient client = new OcrClient() {
             @Override public List<OcrBlockResult> extractBlocks(String pdfUrl, ScriptType scriptType) { return List.of(); }
             @Override public TrainingResult trainModel(byte[] zip) { return null; }
+            @Override public TrainingResult segtrainModel(byte[] zip) { return null; }
         };
 
         List<OcrStreamEvent> events = new ArrayList<>();