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_test.go

@@ -189,11 +189,11 @@ func (m *mockChannelAuthCacheInvalidator) InvalidateAuthCacheByGroupID(_ context
 // ---------------------------------------------------------------------------
 
 func newTestChannelService(repo *mockChannelRepository) *ChannelService {
-	return NewChannelService(repo, nil, nil)
+	return NewChannelService(repo, nil, nil, nil)
 }
 
 func newTestChannelServiceWithAuth(repo *mockChannelRepository, auth *mockChannelAuthCacheInvalidator) *ChannelService {
-	return NewChannelService(repo, nil, auth)
+	return NewChannelService(repo, nil, auth, nil)
 }
 
 // makeStandardRepo returns a repo that serves one active channel with anthropic pricing