feat(ocr): add default streamBlocks method to OcrClient interface

The default method synthesizes Start/Page/Done events from
extractBlocks() results, providing backward compatibility for
implementations that don't support streaming natively.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

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

@@ -2,8 +2,34 @@ package org.raddatz.familienarchiv.service;
 
 import org.raddatz.familienarchiv.model.ScriptType;
 
+import java.util.ArrayList;
+import java.util.LinkedHashMap;
 import java.util.List;
+import java.util.function.Consumer;
 
 public interface OcrClient {
     List<OcrBlockResult> extractBlocks(String pdfUrl, ScriptType scriptType);
+
+    /**
+     * Stream OCR results page-by-page via NDJSON. Implementations should override
+     * this method. The default exists only for backward compatibility during migration
+     * — it calls extractBlocks() and synthesizes events from the collected result.
+     */
+    default void streamBlocks(String pdfUrl, ScriptType scriptType, Consumer<OcrStreamEvent> handler) {
+        List<OcrBlockResult> allBlocks = extractBlocks(pdfUrl, scriptType);
+
+        LinkedHashMap<Integer, List<OcrBlockResult>> byPage = new LinkedHashMap<>();
+        for (OcrBlockResult block : allBlocks) {
+            byPage.computeIfAbsent(block.pageNumber(), k -> new ArrayList<>()).add(block);
+        }
+
+        int totalPages = byPage.isEmpty() ? 0 : byPage.keySet().stream().mapToInt(i -> i).max().orElse(0) + 1;
+        handler.accept(new OcrStreamEvent.Start(totalPages));
+
+        for (var entry : byPage.entrySet()) {
+            handler.accept(new OcrStreamEvent.Page(entry.getKey(), entry.getValue()));
+        }
+
+        handler.accept(new OcrStreamEvent.Done(allBlocks.size(), 0));
+    }
 }

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

@@ -0,0 +1,55 @@
+package org.raddatz.familienarchiv.service;
+
+import org.junit.jupiter.api.Test;
+import org.raddatz.familienarchiv.model.ScriptType;
+
+import java.util.ArrayList;
+import java.util.List;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+class OcrClientDefaultStreamTest {
+
+    @Test
+    void defaultStreamBlocksSynthesizesEventsFromExtractBlocks() {
+        OcrClient client = (pdfUrl, scriptType) -> List.of(
+                new OcrBlockResult(0, 0.1, 0.1, 0.8, 0.04, null, "Line 1"),
+                new OcrBlockResult(0, 0.1, 0.2, 0.8, 0.04, null, "Line 2"),
+                new OcrBlockResult(1, 0.1, 0.1, 0.8, 0.04, null, "Line 3"));
+
+        List<OcrStreamEvent> events = new ArrayList<>();
+        client.streamBlocks("http://test", ScriptType.TYPEWRITER, events::add);
+
+        assertThat(events).hasSize(4);
+        assertThat(events.get(0)).isInstanceOf(OcrStreamEvent.Start.class);
+        assertThat(((OcrStreamEvent.Start) events.get(0)).totalPages()).isEqualTo(2);
+
+        assertThat(events.get(1)).isInstanceOf(OcrStreamEvent.Page.class);
+        var page0 = (OcrStreamEvent.Page) events.get(1);
+        assertThat(page0.pageNumber()).isEqualTo(0);
+        assertThat(page0.blocks()).hasSize(2);
+
+        assertThat(events.get(2)).isInstanceOf(OcrStreamEvent.Page.class);
+        var page1 = (OcrStreamEvent.Page) events.get(2);
+        assertThat(page1.pageNumber()).isEqualTo(1);
+        assertThat(page1.blocks()).hasSize(1);
+
+        assertThat(events.get(3)).isInstanceOf(OcrStreamEvent.Done.class);
+        var done = (OcrStreamEvent.Done) events.get(3);
+        assertThat(done.totalBlocks()).isEqualTo(3);
+        assertThat(done.skippedPages()).isEqualTo(0);
+    }
+
+    @Test
+    void defaultStreamBlocksHandlesEmptyResults() {
+        OcrClient client = (pdfUrl, scriptType) -> List.of();
+
+        List<OcrStreamEvent> events = new ArrayList<>();
+        client.streamBlocks("http://test", ScriptType.TYPEWRITER, events::add);
+
+        assertThat(events).hasSize(2);
+        assertThat(events.get(0)).isInstanceOf(OcrStreamEvent.Start.class);
+        assertThat(((OcrStreamEvent.Start) events.get(0)).totalPages()).isEqualTo(0);
+        assertThat(events.get(1)).isInstanceOf(OcrStreamEvent.Done.class);
+    }
+}