fix: TLS连接池复用 + 禁言检测 + 降级回复静默

- llm/openai.go: 移除DisableKeepAlives，启用连接池(100idle/20perhost/90s)
  修复频繁TLS握手超时问题
- qq/adapter.go: 监听group_ban通知，发送前检查禁言状态
  防止向被禁言群发消息导致风控
- cmd/main.go: 降级回复仅管理员私聊，群聊静默失败避免怪话
- protocol.go: OBv11Message新增Duration字段
- bridge/router.go: 处理noticeToUnified返回nil(内部消费的通知)

backend/platform-bridge/internal/bridge/router.go

@@ -139,6 +139,10 @@ func (r *PlatformRouter) RouteMessage(adapterKey string, rawMsg interface{}) (*U
 	if err != nil {
 		return nil, fmt.Errorf("convert to unified: %w", err)
 	}
+	// nil means the notice was handled internally (e.g., mute tracking) — no dispatch needed.
+	if unified == nil {
+		return nil, nil
+	}
 
 	// Preserve original platform UID before identity mapping.
 	unified.OriginalSenderUID = unified.SenderID