feat(ocr): wire SenderModelService into OcrAsyncRunner; stage missing foundational files

OcrAsyncRunner now passes the per-sender model path to streamBlocks for
HANDWRITING_KURRENT documents. processDocument replaced extractBlocks
with streamBlocks + AtomicReference, removing the unchecked raw-array
pattern.

Also stages all previously uncommitted foundational files for this
feature: SenderModel entity, SenderModelRepository, Flyway migrations
V40/V41, updated OcrClient/RestClientOcrClient streaming API,
TrainingDataExportService.exportForSender, TranscriptionService Kurrent
hook, application.yaml OCR config, and frontend i18n/test additions.

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

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

@@ -21,6 +21,7 @@ class OcrClientDefaultStreamTest {
             }
             @Override public TrainingResult trainModel(byte[] zip) { return null; }
             @Override public TrainingResult segtrainModel(byte[] zip) { return null; }
+            @Override public TrainingResult trainSenderModel(byte[] zip, String path) { return null; }
         };
 
         List<OcrStreamEvent> events = new ArrayList<>();
@@ -52,6 +53,7 @@ class OcrClientDefaultStreamTest {
             @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; }
+            @Override public TrainingResult trainSenderModel(byte[] zip, String path) { return null; }
         };
 
         List<OcrStreamEvent> events = new ArrayList<>();

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

@@ -22,6 +22,7 @@ class TranscriptionServiceGuidedTest {
     AnnotationRepository annotationRepository;
     AnnotationService annotationService;
     DocumentService documentService;
+    SenderModelService senderModelService;
     TranscriptionService service;
 
     UUID docId = UUID.randomUUID();
@@ -35,9 +36,10 @@ class TranscriptionServiceGuidedTest {
         annotationRepository = mock(AnnotationRepository.class);
         annotationService = mock(AnnotationService.class);
         documentService = mock(DocumentService.class);
+        senderModelService = mock(SenderModelService.class);
 
         service = new TranscriptionService(blockRepository, versionRepository,
-                annotationRepository, annotationService, documentService);
+                annotationRepository, annotationService, documentService, senderModelService);
 
         when(blockRepository.save(any())).thenAnswer(inv -> inv.getArgument(0));
         when(versionRepository.save(any())).thenAnswer(inv -> inv.getArgument(0));