Fix connection logs: add background status.log sync task

ConnLogs were never populated because AppendConnLog was never called.
Added StartStatusSync() goroutine that runs every 10 seconds:

1. Iterates all running instances
2. Reads status.log via ParseStatus (status-version 3)
3. For each CLIENT_LIST entry:
   - If no active ConnLog exists → AppendConnLog (new connection)
   - If active ConnLog exists → UpdateActiveConnStats (traffic)
4. For active ConnLogs with no matching CLIENT_LIST → CloseActiveConn

Store additions:
  ListActiveConns(instanceID)  — returns all unclosed connections
  UpdateActiveConnStats()      — updates bytes in/out without flush

StartStatusSync is called from main.go right after Service creation.
UpdateActiveConnStats deliberately skips flush() to avoid writing
db.json every 10 seconds; stats are persisted on disconnect.

backend/internal/store/store.go

@@ -287,6 +287,35 @@ func (s *Store) CloseActiveConn(instanceID, commonName string, at time.Time) err
 	return nil
 }
 
+// ListActiveConns 返回指定实例所有未断开的连接。
+func (s *Store) ListActiveConns(instanceID string) []model.ConnectionLog {
+	s.mu.RLock()
+	defer s.mu.RUnlock()
+	var out []model.ConnectionLog
+	for i := len(s.data.ConnLogs) - 1; i >= 0; i-- {
+		c := s.data.ConnLogs[i]
+		if c.InstanceID == instanceID && c.DisconnectedAt == nil {
+			out = append(out, c)
+		}
+	}
+	return out
+}
+
+// UpdateActiveConnStats 更新活跃连接的流量统计。
+func (s *Store) UpdateActiveConnStats(instanceID, commonName string, bytesIn, bytesOut int64) error {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+	for i := len(s.data.ConnLogs) - 1; i >= 0; i-- {
+		c := &s.data.ConnLogs[i]
+		if c.InstanceID == instanceID && c.CommonName == commonName && c.DisconnectedAt == nil {
+			c.BytesIn = bytesIn
+			c.BytesOut = bytesOut
+			return nil // 不 flush,避免高频写盘
+		}
+	}
+	return nil
+}
+
 // ---- Backups ----
 
 func (s *Store) ListBackups() []model.Backup {