Add post change log tracking

backend/src/main/java/com/openisle/model/PostContentChangeLog.java

@@ -0,0 +1,21 @@
+package com.openisle.model;
+
+import jakarta.persistence.Column;
+import jakarta.persistence.Entity;
+import jakarta.persistence.Table;
+import lombok.Getter;
+import lombok.NoArgsConstructor;
+import lombok.Setter;
+
+@Getter
+@Setter
+@NoArgsConstructor
+@Entity
+@Table(name = "post_content_change_logs")
+public class PostContentChangeLog extends PostChangeLog {
+    @Column(name = "old_content", columnDefinition = "LONGTEXT")
+    private String oldContent;
+
+    @Column(name = "new_content", columnDefinition = "LONGTEXT")
+    private String newContent;
+}