Implement profile page and related backend APIs

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

@@ -4,6 +4,7 @@ import jakarta.persistence.*;
 import lombok.Getter;
 import lombok.NoArgsConstructor;
 import lombok.Setter;
+import java.time.LocalDateTime;
 
 import com.openisle.model.Role;
 
@@ -43,4 +44,12 @@ public class User {
     @Enumerated(EnumType.STRING)
     @Column(nullable = false)
     private Role role = Role.USER;
+
+    @Column(nullable = false)
+    private LocalDateTime createdAt;
+
+    @PrePersist
+    protected void onCreate() {
+        this.createdAt = LocalDateTime.now();
+    }
 }