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/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'
       }