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/service/channel_test.go

@@ -637,3 +637,34 @@ func TestSupportedModels_EmptyPlatformMapping(t *testing.T) {
 	}
 	require.Empty(t, ch.SupportedModels())
 }
+
+func TestSupportedModels_ExactKeyUsesPricedCaseWhenAvailable(t *testing.T) {
+	// mapping key uses uppercase, pricing uses lowercase — pricing's case should win.
+	ch := &Channel{
+		ModelPricing: []ChannelModelPricing{
+			{ID: 1, Platform: "openai", Models: []string{"gpt-4o"}},
+		},
+		ModelMapping: map[string]map[string]string{
+			"openai": {"GPT-4o": "gpt-4o"},
+		},
+	}
+	got := ch.SupportedModels()
+	require.Len(t, got, 1)
+	require.Equal(t, "gpt-4o", got[0].Name) // pricing's case wins
+}
+
+func TestSupportedModels_AsteriskOnlyMappingExpandsAllPriced(t *testing.T) {
+	// 映射 key 为单独的 "*"：前缀为空 → 命中该平台所有定价模型（透传场景）。
+	ch := &Channel{
+		ModelPricing: []ChannelModelPricing{
+			{ID: 1, Platform: "openai", Models: []string{"gpt-4o", "gpt-4o-mini"}},
+		},
+		ModelMapping: map[string]map[string]string{
+			"openai": {"*": "gpt-4o"},
+		},
+	}
+	got := ch.SupportedModels()
+	require.Len(t, got, 2)
+	names := []string{got[0].Name, got[1].Name}
+	require.ElementsMatch(t, []string{"gpt-4o", "gpt-4o-mini"}, names)
+}