feat: add user leveling and experience system

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

@@ -4,6 +4,7 @@ import jakarta.persistence.*;
 import lombok.Getter;
 import lombok.NoArgsConstructor;
 import lombok.Setter;
+import org.hibernate.annotations.CreationTimestamp;
 
 /**
  * Reaction entity representing a user's reaction to a post or comment.
@@ -37,4 +38,9 @@ public class Reaction {
     @ManyToOne(fetch = FetchType.LAZY)
     @JoinColumn(name = "comment_id")
     private Comment comment;
+
+    @CreationTimestamp
+    @Column(nullable = false, updatable = false,
+            columnDefinition = "DATETIME(6) DEFAULT CURRENT_TIMESTAMP(6)")
+    private java.time.LocalDateTime createdAt;
 }

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

@@ -39,6 +39,9 @@ public class User {
 
     private String avatar;
 
+    @Column(nullable = false)
+    private int experience = 0;
+
     @Column(length = 1000)
     private String introduction;