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).

backend/internal/handler/admin/available_channel_handler.go

@@ -45,9 +45,9 @@ type availableChannelResponse struct {
 	SupportedModels    []supportedModelResponse `json:"supported_models"`
 }
 
-// AvailableChannelToAdminResponse 将 service 层的 AvailableChannel 转为管理员 DTO。
-// 导出供同 package 的复用；也用于构造测试 fixture。
-func AvailableChannelToAdminResponse(ch service.AvailableChannel) availableChannelResponse {
+// availableChannelToAdminResponse 将 service 层的 AvailableChannel 转为管理员 DTO。
+// 同 package 内复用；也用于构造测试 fixture。
+func availableChannelToAdminResponse(ch service.AvailableChannel) availableChannelResponse {
 	groups := make([]availableGroupResponse, 0, len(ch.Groups))
 	for _, g := range ch.Groups {
 		groups = append(groups, availableGroupResponse{ID: g.ID, Name: g.Name, Platform: g.Platform})
@@ -66,16 +66,12 @@ func AvailableChannelToAdminResponse(ch service.AvailableChannel) availableChann
 			Pricing:  pricing,
 		})
 	}
-	billingSource := ch.BillingModelSource
-	if billingSource == "" {
-		billingSource = service.BillingModelSourceChannelMapped
-	}
 	return availableChannelResponse{
 		ID:                 ch.ID,
 		Name:               ch.Name,
 		Description:        ch.Description,
 		Status:             ch.Status,
-		BillingModelSource: billingSource,
+		BillingModelSource: ch.BillingModelSource,
 		RestrictModels:     ch.RestrictModels,
 		Groups:             groups,
 		SupportedModels:    models,
@@ -93,7 +89,7 @@ func (h *AvailableChannelHandler) List(c *gin.Context) {
 
 	out := make([]availableChannelResponse, 0, len(channels))
 	for _, ch := range channels {
-		out = append(out, AvailableChannelToAdminResponse(ch))
+		out = append(out, availableChannelToAdminResponse(ch))
 	}
 	response.Success(c, gin.H{"items": out})
 }