feat: opensearch init

backend/src/main/java/com/openisle/search/SearchDocument.java

@@ -0,0 +1,16 @@
+package com.openisle.search;
+
+import java.time.Instant;
+import java.util.List;
+
+public record SearchDocument(
+  String type,
+  Long entityId,
+  String title,
+  String content,
+  String author,
+  String category,
+  List<String> tags,
+  Long postId,
+  Instant createdAt
+) {}