Add post change log tracking

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

@@ -0,0 +1,19 @@
+package com.openisle.model;
+
+import jakarta.persistence.Entity;
+import jakarta.persistence.Table;
+import lombok.Getter;
+import lombok.NoArgsConstructor;
+import lombok.Setter;
+
+import java.time.LocalDateTime;
+
+@Getter
+@Setter
+@NoArgsConstructor
+@Entity
+@Table(name = "post_pinned_change_logs")
+public class PostPinnedChangeLog extends PostChangeLog {
+    private LocalDateTime oldPinnedAt;
+    private LocalDateTime newPinnedAt;
+}