first implementation

backend/src/main/java/com/aiteacher/chat/ChatSession.java

@@ -0,0 +1,48 @@
+package com.aiteacher.chat;
+
+import jakarta.persistence.*;
+import java.time.Instant;
+import java.util.UUID;
+
+@Entity
+@Table(name = "chat_session")
+public class ChatSession {
+
+    @Id
+    @GeneratedValue(strategy = GenerationType.UUID)
+    private UUID id;
+
+    @Column(name = "topic_id", length = 100)
+    private String topicId;
+
+    @Column(name = "created_at", nullable = false)
+    private Instant createdAt;
+
+    public ChatSession() {
+    }
+
+    public ChatSession(String topicId) {
+        this.topicId = topicId;
+        this.createdAt = Instant.now();
+    }
+
+    public UUID getId() {
+        return id;
+    }
+
+    public String getTopicId() {
+        return topicId;
+    }
+
+    public void setTopicId(String topicId) {
+        this.topicId = topicId;
+    }
+
+    public Instant getCreatedAt() {
+        return createdAt;
+    }
+
+    public void setCreatedAt(Instant createdAt) {
+        this.createdAt = createdAt;
+    }
+}