chore(channels): drop admin-side available channels view

Remove the admin-side "Available Channels" aggregate view — admins
already see full channel configuration (groups, pricing, model
mappings) in the channel edit dialog, making a read-only admin
aggregate view redundant. The user-side "可用渠道" remains.

Backend:
- Delete handler/admin/available_channel_handler.go (+ test)
- Drop AdminHandlers.AvailableChannel field and wire injection
- Remove /admin/channels/available route

Frontend:
- Delete views/admin/AvailableChannelsView.vue
- Drop /admin/available-channels router entry
- Strip AvailableChannel types + listAvailable from api/admin/channels.ts

frontend/src/api/admin/channels.ts

@@ -164,42 +164,5 @@ export async function getModelDefaultPricing(model: string): Promise<ModelDefaul
   return data
 }
 
-// --- Available channels (聚合视图：渠道 + 分组 + 支持模型) ---
-
-export interface AvailableGroupRef {
-  id: number
-  name: string
-  platform: string
-}
-
-export interface SupportedModel {
-  name: string
-  platform: string
-  pricing: ChannelModelPricing | null
-}
-
-export interface AvailableChannel {
-  id: number
-  name: string
-  description: string
-  status: ChannelStatus
-  billing_model_source: BillingModelSource
-  restrict_models: boolean
-  groups: AvailableGroupRef[]
-  supported_models: SupportedModel[]
-}
-
-interface AvailableChannelsResponse {
-  items: AvailableChannel[]
-}
-
-/** 列出所有可用渠道（含关联分组与支持模型） */
-export async function listAvailable(options?: { signal?: AbortSignal }): Promise<AvailableChannel[]> {
-  const { data } = await apiClient.get<AvailableChannelsResponse>('/admin/channels/available', {
-    signal: options?.signal
-  })
-  return data.items
-}
-
-const channelsAPI = { list, getById, create, update, remove, getModelDefaultPricing, listAvailable }
+const channelsAPI = { list, getById, create, update, remove, getModelDefaultPricing }
 export default channelsAPI