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_available_test.go

@@ -75,7 +75,7 @@ func newAvailableChannelService(channels []Channel, groupRepo GroupRepository) *
 	repo := &mockChannelRepository{
 		listAllFn: func(ctx context.Context) ([]Channel, error) { return channels, nil },
 	}
-	return NewChannelService(repo, groupRepo, nil)
+	return NewChannelService(repo, groupRepo, nil, nil)
 }
 
 func TestListAvailable_EmptyActiveGroups_NoGroupsAttached(t *testing.T) {
@@ -134,7 +134,7 @@ func TestListAvailable_ListAllErrorPropagates(t *testing.T) {
 		listAllFn: func(ctx context.Context) ([]Channel, error) { return nil, sentinel },
 	}
 	groupRepo := &stubGroupRepoForAvailable{}
-	svc := NewChannelService(repo, groupRepo, nil)
+	svc := NewChannelService(repo, groupRepo, nil, nil)
 	out, err := svc.ListAvailable(context.Background())
 	require.Nil(t, out)
 	require.ErrorIs(t, err, sentinel)