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/adapter/qq/protocol.go

@@ -27,6 +27,7 @@ type OBv11Message struct {
 
 	// Notice fields.
 	NoticeType string `json:"notice_type"`
+	Duration   int64  `json:"duration"` // group_ban: mute duration in seconds (0 = indefinite)
 
 	// Poke detail (sub_type === "poke").
 	PokeDetail *OBv11PokeDetail `json:"poke_detail,omitempty"`