fix: restoreContext 从会话历史取最近活动时间，不再用记忆条目时间

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

@@ -533,31 +533,20 @@ func NewThinker(
 	}
 }
 
-// restoreContext recovers the last user activity time from persistent storage.
+// restoreContext recovers the last user activity time from conversation history (DB).
 func (t *Thinker) restoreContext() {
-	if t.memClient == nil {
+	if t.convStore == nil {
 		return
 	}
-	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
-	defer cancel()
-	defer func() { t.muLock(); t.thinkCancel = nil; t.muUnlock() }()
-
-	memories, err := t.memClient.Query(ctx, model.MemoryQuery{
-		UserID: t.adminUserID,
-		Limit:  5,
-	})
-	if err != nil || len(memories) == 0 {
-		return
-	}
-
-	// Find the most recent memory/activity timestamp.
+	// 从已恢复的会话历史中获取最后一条用户消息的时间
+	history := t.convStore.GetHistory(t.adminSessionID, 5)
 	var latest time.Time
-	for _, m := range memories {
-		if m.CreatedAt.After(latest) {
-			latest = m.CreatedAt
+	for i := len(history) - 1; i >= 0; i-- {
+		if history[i].Role == model.RoleUser && !history[i].Timestamp.IsZero() {
+			latest = history[i].Timestamp
+			break
 		}
 	}
-	// Only use if reasonably recent (within 24h). Old memories don't represent user activity.
 	if !latest.IsZero() && time.Since(latest) < 24*time.Hour {
 		t.muLock()
 		t.lastUserMessage = latest
@@ -565,7 +554,7 @@ func (t *Thinker) restoreContext() {
 		t.muUnlock()
 		log.Printf("[后台思考] 上下文已恢复: 最近活动 %v 前", time.Since(latest).Round(time.Second))
 	} else if !latest.IsZero() {
-		log.Printf("[后台思考] 上下文恢复跳过: 最近记忆 %v 前(>24h)，使用默认值", time.Since(latest).Round(time.Second))
+		log.Printf("[后台思考] 上下文恢复跳过: 最近消息 %v 前(>24h)，使用默认值", time.Since(latest).Round(time.Second))
 	}
 }