Fix default creation timestamps

src/main/java/com/openisle/model/Comment.java

@@ -4,6 +4,7 @@ import jakarta.persistence.*;
 import lombok.Getter;
 import lombok.NoArgsConstructor;
 import lombok.Setter;
+import org.hibernate.annotations.CreationTimestamp;
 
 import java.time.LocalDateTime;
 
@@ -20,7 +21,9 @@ public class Comment {
     @Column(nullable = false, columnDefinition = "TEXT")
     private String content;
 
-    @Column(nullable = false)
+    @CreationTimestamp
+    @Column(nullable = false, updatable = false,
+            columnDefinition = "DATETIME(6) DEFAULT CURRENT_TIMESTAMP(6)")
     private LocalDateTime createdAt;
 
     @ManyToOne(fetch = FetchType.LAZY, optional = false)
@@ -35,8 +38,4 @@ public class Comment {
     @JoinColumn(name = "parent_id")
     private Comment parent;
 
-    @PrePersist
-    protected void onCreate() {
-        this.createdAt = LocalDateTime.now();
-    }
 }