feat: 重启后恢复上下文 — 从记忆服务查询最近活动时间

- 新增 restoreContext() 在 Start() 时查询持久化记忆
- 取最近记忆的 CreatedAt 设为 lastUserMessage
- 避免重启后思考出现"已经一天没说话了"

Co-Authored-By: Claude <noreply@anthropic.com>

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

@@ -490,7 +490,7 @@ func NewThinker(
 		adminNickname:    adminNickname,
 		memClient:        memClient,
 		pendingThoughts:     make([]*PendingThought, 0),
-		lastUserMessage:     time.Now(),
+		lastUserMessage:     time.Now().Add(-5 * time.Minute), // overwritten by restoreContext
 		stopCh:              make(chan struct{}),
 		chain:               NewThinkChain(10),
 		autoToolPolicy:      DefaultAutonomousToolPolicy(),
@@ -501,6 +501,37 @@ func NewThinker(
 	}
 }
 
+// restoreContext recovers the last user activity time from persistent storage.
+func (t *Thinker) restoreContext() {
+	if t.memClient == nil {
+		return
+	}
+	ctx, cancel := context.WithTimeout(context.Background(), 10*time.Second)
+	defer cancel()
+
+	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.
+	var latest time.Time
+	for _, m := range memories {
+		if m.CreatedAt.After(latest) {
+			latest = m.CreatedAt
+		}
+	}
+	if !latest.IsZero() {
+		t.mu.Lock()
+		t.lastUserMessage = latest
+		t.mu.Unlock()
+		log.Printf("[后台思考] 上下文已恢复: 最近活动 %v 前", time.Since(latest).Round(time.Second))
+	}
+}
+
 // Start 初始化后台思考器
 //
 // 双模式触发：
@@ -512,6 +543,9 @@ func (t *Thinker) Start() {
 		return
 	}
 
+	// 恢复上下文：从持久化存储查询最近活动时间，避免重启后"失忆"
+	t.restoreContext()
+
 	// 初始化静默检测定时器（但不启动，等第一次用户消息后启动）
 	if t.silenceTimeout > 0 {
 		t.silenceTimer = time.NewTimer(t.silenceTimeout)