feat(ocr): add OcrStreamEvent sealed interface with Start/Page/Error/Done records

Defines the event types for NDJSON streaming OCR. Uses Java 21 sealed
interface with record subtypes for exhaustive pattern matching in the
consumer.

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

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

@@ -0,0 +1,14 @@
+package org.raddatz.familienarchiv.service;
+
+import java.util.List;
+
+public sealed interface OcrStreamEvent {
+
+    record Start(int totalPages) implements OcrStreamEvent {}
+
+    record Page(int pageNumber, List<OcrBlockResult> blocks) implements OcrStreamEvent {}
+
+    record Error(int pageNumber, String message) implements OcrStreamEvent {}
+
+    record Done(int totalBlocks, int skippedPages) implements OcrStreamEvent {}
+}