feat(ocr): add Preprocessing NDJSON event to Java stream pipeline

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

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

@@ -82,6 +82,10 @@ public class OcrAsyncRunner {
                             ocrJobDocumentRepository.save(jobDoc);
                         }
                     }
+                    case OcrStreamEvent.Preprocessing preprocessing -> {
+                        updateProgress(job, "PREPROCESSING_PAGE:" + preprocessing.pageNumber()
+                                + ":" + totalPages.get());
+                    }
                     case OcrStreamEvent.Page page -> {
                         for (OcrBlockResult block : page.blocks()) {
                             createSingleBlock(documentId, block, userId,

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

@@ -6,6 +6,8 @@ public sealed interface OcrStreamEvent {
 
     record Start(int totalPages) implements OcrStreamEvent {}
 
+    record Preprocessing(int pageNumber) implements OcrStreamEvent {}
+
     record Page(int pageNumber, List<OcrBlockResult> blocks) implements OcrStreamEvent {}
 
     record Error(int pageNumber, String message) implements OcrStreamEvent {}

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

@@ -232,6 +232,8 @@ public class RestClientOcrClient implements OcrClient, OcrHealthClient {
                 switch (type) {
                     case "start" -> handler.accept(
                             new OcrStreamEvent.Start(node.path("totalPages").asInt()));
+                    case "preprocessing" -> handler.accept(
+                            new OcrStreamEvent.Preprocessing(node.path("pageNumber").asInt()));
                     case "page" -> {
                         int pageNumber = node.path("pageNumber").asInt();
                         List<OcrBlockResult> blocks = NDJSON_MAPPER.convertValue(