fix: 修复三个空白页面 + 许可证页面重构

- 修复发现页空白：后端 discover.go nil→[] Match，前端 discover.ts 加 ?? []
- 修复下载客户端页空白：后端 download_clients.go nil→[]model.DownloadClient，前端加 ?? []
- 修复通知渠道页空白：后端 notify_channels.go nil→[]struct{}，前端加 ?? []
- 许可证页面重构：移除生成密钥功能，对接 LicenseServer 绑定/状态模式
- 所有变更通过 Go build + npm run build 零错误验证

web/src/api/notify_channels.ts

@@ -13,7 +13,7 @@ export interface NotifyChannelInput {
 // notifyChannelsAPI wraps the admin /admin/notify/channels surface.
 export const notifyChannelsAPI = {
   list: () =>
-    api.get<NotifyChannel[]>('/admin/notify/channels').then((r) => r.data),
+    api.get<NotifyChannel[]>('/admin/notify/channels').then((r) => r.data ?? []),
 
   create: (input: NotifyChannelInput) =>
     api.post<NotifyChannel>('/admin/notify/channels', input).then((r) => r.data),