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_test.go

@@ -12,13 +12,13 @@ import (
 
 func TestAvailableChannelToAdminResponse_IncludesFullDTO(t *testing.T) {
 	// 管理员视图应包含 id / status / billing_model_source / restrict_models 等
-	// 管理字段；BillingModelSource 为空时应默认回填 channel_mapped。
+	// 管理字段；mapper 是纯透传，BillingModelSource 的默认回填由 service 层负责。
 	input := service.AvailableChannel{
 		ID:                 42,
 		Name:               "ch",
 		Description:        "d",
 		Status:             service.StatusActive,
-		BillingModelSource: "", // 验证默认值填充
+		BillingModelSource: service.BillingModelSourceChannelMapped,
 		RestrictModels:     true,
 		Groups: []service.AvailableGroupRef{
 			{ID: 1, Name: "g1", Platform: "anthropic"},
@@ -28,7 +28,7 @@ func TestAvailableChannelToAdminResponse_IncludesFullDTO(t *testing.T) {
 		},
 	}
 
-	resp := AvailableChannelToAdminResponse(input)
+	resp := availableChannelToAdminResponse(input)
 	require.Equal(t, int64(42), resp.ID)
 	require.Equal(t, "ch", resp.Name)
 	require.Equal(t, service.StatusActive, resp.Status)
@@ -52,6 +52,6 @@ func TestAvailableChannelToAdminResponse_PreservesExplicitBillingSource(t *testi
 	input := service.AvailableChannel{
 		BillingModelSource: service.BillingModelSourceUpstream,
 	}
-	resp := AvailableChannelToAdminResponse(input)
+	resp := availableChannelToAdminResponse(input)
 	require.Equal(t, service.BillingModelSourceUpstream, resp.BillingModelSource)
 }