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/discover.ts

@@ -15,7 +15,7 @@ export interface DiscoverItem extends Partial<Media> {
 
 export const discoverAPI = {
   trending: () =>
-    api.get<{ items: DiscoverItem[] }>('/discover/trending').then((r) => r.data.items),
+    api.get<{ items: DiscoverItem[] }>('/discover/trending').then((r) => r.data.items ?? []),
   popular: () =>
-    api.get<{ items: DiscoverItem[] }>('/discover/popular').then((r) => r.data.items),
+    api.get<{ items: DiscoverItem[] }>('/discover/popular').then((r) => r.data.items ?? []),
 }

web/src/api/download_clients.ts

@@ -28,7 +28,7 @@ export interface DownloadClientInput {
 
 export const downloadClientsAPI = {
   list: () =>
-    api.get<DownloadClient[]>('/admin/download/clients').then((r) => r.data),
+    api.get<DownloadClient[]>('/admin/download/clients').then((r) => r.data ?? []),
 
   create: (input: DownloadClientInput) =>
     api.post<DownloadClient>('/admin/download/clients', input).then((r) => r.data),

web/src/api/license.ts

@@ -1,71 +1,59 @@
 import { api } from './client'
 
-// LicenseKey + LicenseActivation mirror the Go model structs.
-export interface LicenseKey {
-  id: string
-  key: string
-  customer?: string
-  plan: string
-  max_activations: number
-  issued_at: string
-  expires_at?: string | null
-  revoked: boolean
-  notes?: string
-  created_at: string
-  updated_at: string
-}
+// ── License Server types (generic, ready for MediaStationLicenseServer integration) ──
 
+/** Response after binding/activating a license. */
 export interface LicenseActivation {
   id: string
   key_id: string
+  /** The license key string (e.g. MS-XXXX-XXXX-XXXX) */
+  key?: string
   device_id: string
   device_name?: string
+  plan?: string
+  max_activations?: number
+  /** ISO8601 — null means perpetual */
+  expires_at?: string | null
+  valid: boolean
   ip?: string
-  unbound_at?: string | null
   heartbeat_at?: string | null
   created_at: string
 }
 
-export interface GenerateKeyInput {
-  customer?: string
-  plan?: string
-  max_activations?: number
-  expires_at?: string // RFC3339; "" or omit for perpetual
-  notes?: string
+/** Status of the currently bound license on this device. */
+export interface LicenseStatus {
+  /** Whether a license is currently active */
+  active: boolean
+  activation?: LicenseActivation
+  /** Error or status message */
+  message?: string
 }
 
+/** Generate a short device fingerprint from browser/OS info. */
+function deviceFingerprint(): string {
+  const nav = window.navigator
+  const parts = [nav.hardwareConcurrency, nav.language, screen.width, screen.height]
+  return btoa(parts.join('|')).slice(0, 32).replace(/[+/=]/g, '')
+}
+
+// ── API methods ──
+
 export const licenseAPI = {
-  generate: (input: GenerateKeyInput) =>
-    api.post<LicenseKey>('/admin/license/generate', input).then((r) => r.data),
-
-  list: () => api.get<LicenseKey[]>('/admin/license/list').then((r) => r.data),
-
-  listActivations: (keyID: string) =>
-    api
-      .get<LicenseActivation[]>(`/admin/license/${keyID}/activations`)
-      .then((r) => r.data),
-
-  revoke: (keyID: string) =>
-    api.post(`/admin/license/${keyID}/revoke`).then((r) => r.data),
-
-  unbind: (activationID: string) =>
-    api.post(`/admin/license/activation/${activationID}/unbind`).then((r) => r.data),
-
-  // Self-service
-  activate: (key: string, deviceID: string, deviceName?: string) =>
+  /** Bind / activate a license key for this device. */
+  bind: (key: string) =>
     api
       .post<LicenseActivation>('/license/activate', {
-        key,
-        device_id: deviceID,
-        device_name: deviceName,
+        key: key.trim(),
+        device_id: deviceFingerprint(),
+        device_name: navigator.platform || 'Web Client',
       })
       .then((r) => r.data),
 
-  status: (keyID: string) =>
-    api
-      .get<{ key: LicenseKey; active_activations: number; valid: boolean }>(
-        '/license/status',
-        { params: { key_id: keyID } },
-      )
-      .then((r) => r.data),
+  /** Get the status of the currently active license. */
+  status: () =>
+    api.get<LicenseStatus>('/license/status').then((r) => r.data),
+
+  /** Refresh the heartbeat for the active license. */
+  heartbeat: () =>
+    api.post('/license/heartbeat').then((r) => r.data),
 }

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),