refactor(channels): consolidate pricing index, tighten types, polish DTOs

Follow-up to the available-channels review pass. No behavior change for
end users; tightens internals based on three independent code reviews.

Backend
- service/channel.go: collapse buildPricingLookup + pricedNamesFor
  into a single platformPricingIndex (byLower + originalCase + ordered
  names), built once per SupportedModels call. Fixes a casing-
  consistency bug where the same logical model appeared with mapping
  case in the exact branch but pricing case in the wildcard branch —
  pricing's original case now wins everywhere.
- service/channel.go: doc that a mapping key of just "*" expands to
  every priced model on the platform (intentional "passthrough all").
- service/channel_available.go: normalize empty BillingModelSource to
  channel_mapped at construction time, removing the same fallback
  duplicated in the admin DTO mapper and the admin Vue template.
- handler/admin/available_channel_handler.go: unexport
  availableChannelToAdminResponse (same-package usage only); mapper
  is now a pure passthrough.
- handler/available_channel_handler.go: drop the middleware2 alias
  (no name collision in this file).

Frontend
- utils/pricing.ts: extract formatScaled, used by SupportedModelChip
  and PricingRow.
- api/admin/channels.ts: re-export BillingMode from constants/channel;
  tighten Channel.status / billing_model_source to ChannelStatus /
  BillingModelSource (and same for AvailableChannel).
- components/channels/AvailableChannelsTable.vue: drop dead
  withDefaults wrapper (loading is required, both call sites pass it).
- views/admin/AvailableChannelsView.vue: drop the redundant
  || BILLING_MODEL_SOURCE_CHANNEL_MAPPED fallback (now applied in
  service layer); remove unused import.
- i18n zh + en: delete unused tierLabel and tokenRange keys from
  both availableChannels.pricing and admin.availableChannels.pricing.

Tests
- New: SupportedModels_ExactKeyUsesPricedCaseWhenAvailable locks the
  pricing-case-wins rule.
- New: SupportedModels_AsteriskOnlyMappingExpandsAllPriced documents
  the "*" expansion rule.
- Admin handler: existing tests adjusted to pass an explicit
  BillingModelSource (default-fill is now exercised by service tests).

frontend/src/api/admin/channels.ts

@@ -4,8 +4,9 @@
  */
 
 import { apiClient } from '../client'
+import type { BillingMode, ChannelStatus, BillingModelSource } from '@/constants/channel'
 
-export type BillingMode = 'token' | 'per_request' | 'image'
+export type { BillingMode } from '@/constants/channel'
 
 export interface PricingInterval {
   id?: number
@@ -46,8 +47,8 @@ export interface Channel {
   id: number
   name: string
   description: string
-  status: string
-  billing_model_source: string // "requested" | "upstream"
+  status: ChannelStatus
+  billing_model_source: BillingModelSource
   restrict_models: boolean
   features_config?: Record<string, unknown>
   group_ids: number[]
@@ -181,8 +182,8 @@ export interface AvailableChannel {
   id: number
   name: string
   description: string
-  status: string
-  billing_model_source: string
+  status: ChannelStatus
+  billing_model_source: BillingModelSource
   restrict_models: boolean
   groups: AvailableGroupRef[]
   supported_models: SupportedModel[]

frontend/src/components/channels/AvailableChannelsTable.vue

@@ -85,18 +85,15 @@ interface Column {
   label: string
 }
 
-withDefaults(
-  defineProps<{
-    columns: Column[]
-    rows: Row[]
-    loading: boolean
-    pricingKeyPrefix: string
-    noPricingLabel: string
-    noModelsLabel: string
-    emptyLabel: string
-  }>(),
-  { loading: false }
-)
+defineProps<{
+  columns: Column[]
+  rows: Row[]
+  loading: boolean
+  pricingKeyPrefix: string
+  noPricingLabel: string
+  noModelsLabel: string
+  emptyLabel: string
+}>()
 
 const slots = useSlots()
 /**

frontend/src/components/channels/PricingRow.vue

@@ -7,6 +7,7 @@
 
 <script setup lang="ts">
 import { computed } from 'vue'
+import { formatScaled } from '@/utils/pricing'
 
 const props = withDefaults(
   defineProps<{
@@ -18,11 +19,6 @@ const props = withDefaults(
   { value: null }
 )
 
-function formatScaled(value: number | null, scale: number): string {
-  if (value == null) return '-'
-  return `$${(value * scale).toPrecision(10).replace(/\.?0+$/, '')}`
-}
-
 const display = computed(() =>
   props.value == null ? '-' : `${formatScaled(props.value, props.scale)} ${props.unit}`
 )

frontend/src/components/channels/SupportedModelChip.vue

@@ -120,6 +120,7 @@
 import { computed } from 'vue'
 import { useI18n } from 'vue-i18n'
 import PricingRow from './PricingRow.vue'
+import { formatScaled } from '@/utils/pricing'
 import {
   BILLING_MODE_TOKEN,
   BILLING_MODE_PER_REQUEST,
@@ -193,11 +194,6 @@ const billingModeLabel = computed(() => {
   }
 })
 
-function formatScaled(value: number | null | undefined, scale: number): string {
-  if (value == null) return '-'
-  return `$${(value * scale).toPrecision(10).replace(/\.?0+$/, '')}`
-}
-
 function formatRange(min: number, max: number | null): string {
   const maxLabel = max == null ? '∞' : String(max)
   return `(${min}, ${maxLabel}]`

frontend/src/i18n/locales/en.ts

@@ -955,8 +955,6 @@ export default {
       imageOutputPrice: 'Image Output',
       perRequestPrice: 'Per Request',
       intervals: 'Tiered Pricing',
-      tierLabel: 'Tier',
-      tokenRange: 'Token Range',
       unitPerMillion: '/ 1M tokens',
       unitPerRequest: '/ request'
     }
@@ -2048,8 +2046,6 @@ export default {
         imageOutputPrice: 'Image Output',
         perRequestPrice: 'Per Request',
         intervals: 'Tiered Pricing',
-        tierLabel: 'Tier',
-        tokenRange: 'Token Range',
         unitPerMillion: '/ 1M tokens',
         unitPerRequest: '/ request'
       }

frontend/src/i18n/locales/zh.ts

@@ -959,8 +959,6 @@ export default {
       imageOutputPrice: '图片输出',
       perRequestPrice: '每次请求',
       intervals: '阶梯定价',
-      tierLabel: '层级',
-      tokenRange: 'Token 区间',
       unitPerMillion: '/ 1M token',
       unitPerRequest: '/ 次'
     }
@@ -2127,8 +2125,6 @@ export default {
         imageOutputPrice: '图片输出',
         perRequestPrice: '每次请求',
         intervals: '阶梯定价',
-        tierLabel: '层级',
-        tokenRange: 'Token 区间',
         unitPerMillion: '/ 1M token',
         unitPerRequest: '/ 次'
       }

frontend/src/utils/pricing.ts

@@ -0,0 +1,13 @@
+/**
+ * formatScaled formats a per-token (or per-request) USD price scaled by `scale`.
+ *
+ *   formatScaled(0.000003, 1_000_000) → "$3"        // per 1M tokens
+ *   formatScaled(0.5,        1)        → "$0.5"      // per request
+ *   formatScaled(null,       1_000_000) → "-"
+ *
+ * Uses toPrecision(10) then strips trailing zeros to avoid IEEE 754 display noise.
+ */
+export function formatScaled(value: number | null, scale: number): string {
+  if (value == null) return '-'
+  return `$${(value * scale).toPrecision(10).replace(/\.?0+$/, '')}`
+}

frontend/src/views/admin/AvailableChannelsView.vue

@@ -59,11 +59,7 @@
 
           <template #cell-billing_model_source="{ row }">
             <span class="text-xs text-gray-700 dark:text-gray-300">
-              {{
-                t(
-                  `admin.availableChannels.billingSource.${row.billing_model_source || BILLING_MODEL_SOURCE_CHANNEL_MAPPED}`
-                )
-              }}
+              {{ t(`admin.availableChannels.billingSource.${row.billing_model_source}`) }}
             </span>
           </template>
         </AvailableChannelsTable>
@@ -82,10 +78,7 @@ import AvailableChannelsTable from '@/components/channels/AvailableChannelsTable
 import channelsAPI, { type AvailableChannel } from '@/api/admin/channels'
 import { useAppStore } from '@/stores/app'
 import { extractApiErrorMessage } from '@/utils/apiError'
-import {
-  CHANNEL_STATUS_ACTIVE,
-  BILLING_MODEL_SOURCE_CHANNEL_MAPPED
-} from '@/constants/channel'
+import { CHANNEL_STATUS_ACTIVE } from '@/constants/channel'
 
 const { t } = useI18n()
 const appStore = useAppStore()