refactor(channels): normalize at cache fill and eliminate frontend as-cast

- channel.go: convert normalizeBillingModelSource into a (*Channel) method for entity cohesion
- channel_service.go: normalize in populateChannelCache so every cache-backed reader (gateway, billing, future endpoints) sees the default; drop the duplicate fallback inside resolveMapping
- table: tighten Row with status?: ChannelStatus / billing_model_source?: BillingModelSource, remove the [key: string]: unknown index signature
- admin view: drop the `as ChannelStatus` / `as BillingModelSource` assertions and add statusStyleOf / billingSourceLabelOf helpers with runtime fallback so unseen values render as "-" instead of crashing

frontend/src/views/admin/AvailableChannelsView.vue

@@ -46,16 +46,16 @@
 
           <template #cell-status="{ row }">
             <span
-              :class="statusStyles[row.status as ChannelStatus].cls"
+              :class="statusStyleOf(row.status).cls"
               class="inline-flex items-center rounded px-2 py-0.5 text-xs font-medium"
             >
-              {{ statusStyles[row.status as ChannelStatus].label }}
+              {{ statusStyleOf(row.status).label }}
             </span>
           </template>
 
           <template #cell-billing_model_source="{ row }">
             <span class="text-xs text-gray-700 dark:text-gray-300">
-              {{ billingSourceLabels[row.billing_model_source as BillingModelSource] }}
+              {{ billingSourceLabelOf(row.billing_model_source) }}
             </span>
           </template>
         </AvailableChannelsTable>
@@ -101,7 +101,7 @@ const columns = computed(() => [
 
 /**
  * 显示样式：i18n label + Tailwind class，按 ChannelStatus 完整穷举。
- * 用 Record<ChannelStatus, ...> 强制未来新增状态时 TS 编译失败，避免遗漏分支。
+ * Record 键类型强制未来新增 ChannelStatus 成员时 TS 编译失败，避免遗漏分支。
  */
 const statusStyles = computed<Record<ChannelStatus, { label: string; cls: string }>>(() => ({
   [CHANNEL_STATUS_ACTIVE]: {
@@ -124,6 +124,19 @@ const billingSourceLabels = computed<Record<BillingModelSource, string>>(() => (
   [BILLING_MODEL_SOURCE_CHANNEL_MAPPED]: t('admin.availableChannels.billingSource.channel_mapped')
 }))
 
+// 运行时兜底：即便 service 层归一化漏点或后端新增未同步的 enum 值传入，
+// 也不会触发 undefined.cls 崩溃；统一降级为 "-"。
+const DEFAULT_STATUS_STYLE = { label: '-', cls: '' }
+const DEFAULT_BILLING_LABEL = '-'
+
+function statusStyleOf(status: ChannelStatus | undefined): { label: string; cls: string } {
+  return status ? statusStyles.value[status] : DEFAULT_STATUS_STYLE
+}
+
+function billingSourceLabelOf(src: BillingModelSource | undefined): string {
+  return src ? billingSourceLabels.value[src] : DEFAULT_BILLING_LABEL
+}
+
 const filteredChannels = computed(() => {
   const q = searchQuery.value.trim().toLowerCase()
   if (!q) return channels.value