fix: LoadFromDB Timestamp 赋值 + restoreContext 查20条消息

backend/ai-core/internal/background/thinker.go

@@ -538,14 +538,20 @@ func (t *Thinker) restoreContext() {
 	if t.convStore == nil {
 		return
 	}
-	// 从已恢复的会话历史中获取最后一条用户消息的时间
+	// 从已恢复的会话历史中获取最后一条消息的时间（优先用户消息）
-	history := t.convStore.GetHistory(t.adminSessionID, 5)
+	history := t.convStore.GetHistory(t.adminSessionID, 20)
 	var latest time.Time
 	for i := len(history) - 1; i >= 0; i-- {
-		if history[i].Role == model.RoleUser && !history[i].Timestamp.IsZero() {
+		if !history[i].Timestamp.IsZero() {
+			if history[i].Role == model.RoleUser {
 				latest = history[i].Timestamp
 				break
 			}
+			// 回退：非 system 消息的时间也可以
+			if latest.IsZero() && history[i].Role != model.RoleSystem {
+				latest = history[i].Timestamp
+			}
+		}
 	}
 	if !latest.IsZero() && time.Since(latest) < 24*time.Hour {
 		t.muLock()

backend/ai-core/internal/context/builder.go

@@ -164,6 +164,7 @@ func (cs *ConversationStore) LoadFromDB(databaseURL, sessionID string, limit int
 		cs.messages[sessionID] = append(cs.messages[sessionID], model.LLMMessage{
 			Role:      role,
 			Content:   content,
+			Timestamp: createdAt,
 		})
 		loaded++
 	}