fix(channels): supported models = mapping ∪ pricing with global LiteLLM fallback

Why: channels with model pricing entries but no model mapping (e.g. azcc with
3 priced claude models, no mapping) were rendering as 未配置模型 in the
'Available Channels' page. The algorithm only iterated ModelMapping and
silently dropped any platform without a mapping entry.

Changes:
- channel.go: SupportedModels now unions mapping + pricing entries.
  For exact mapping src → target, pricing is looked up by target (the actually
  billed name), not by src.
- channel_available.go: ListAvailable enriches each entry with nil pricing
  via PricingService.GetModelPricing (global LiteLLM fallback) so the popover
  always shows a price.
- channel_service.go: NewChannelService takes *PricingService as 4th param.
- channel_test.go: rewrote 4 tests that froze the old mapping-only semantics;
  added pricing-only / mapping-target / target-missing coverage.

backend/internal/service/channel_service.go

@@ -143,17 +143,21 @@ type ChannelService struct {
 	repo                 ChannelRepository
 	groupRepo            GroupRepository
 	authCacheInvalidator APIKeyAuthCacheInvalidator
+	pricingService       *PricingService // 用于「可用渠道」展示时回落到全局定价；可为 nil（测试场景）
 
 	cache   atomic.Value // *channelCache
 	cacheSF singleflight.Group
 }
 
-// NewChannelService 创建渠道服务实例
-func NewChannelService(repo ChannelRepository, groupRepo GroupRepository, authCacheInvalidator APIKeyAuthCacheInvalidator) *ChannelService {
+// NewChannelService 创建渠道服务实例。
+// pricingService 仅供 ListAvailable 在渠道未配置定价时回落到全局 LiteLLM 数据；
+// 计费热路径走独立的 ModelPricingResolver，与此参数无关。可传 nil。
+func NewChannelService(repo ChannelRepository, groupRepo GroupRepository, authCacheInvalidator APIKeyAuthCacheInvalidator, pricingService *PricingService) *ChannelService {
 	s := &ChannelService{
 		repo:                 repo,
 		groupRepo:            groupRepo,
 		authCacheInvalidator: authCacheInvalidator,
+		pricingService:       pricingService,
 	}
 	return s
 }