refactor(channels): tighten types and error paths per second review

- service: drop groupRepo nil guard (DI must inject), switch SupportedModels to SliceStable to match doc
- frontend: reuse user-side DTO types in SupportedModelChip/AvailableChannelsTable instead of duplicating shapes; narrow admin statusLabel param to ChannelStatus
- tests: replace nil-groupRepo case with ListAll/ListActive error propagation and BillingModelSource default-backfill coverage

frontend/src/components/channels/AvailableChannelsTable.vue

@@ -61,6 +61,7 @@ import { computed, useSlots } from 'vue'
 import DataTable from '@/components/common/DataTable.vue'
 import Icon from '@/components/icons/Icon.vue'
 import SupportedModelChip from './SupportedModelChip.vue'
+import type { UserSupportedModelPricing } from '@/api/channels'
 
 interface GroupRef {
   id: number
@@ -75,7 +76,7 @@ interface Row {
   supported_models: Array<{
     name: string
     platform: string
-    pricing: unknown | null
+    pricing: UserSupportedModelPricing | null
   }>
   [key: string]: unknown
 }