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/model_pricing_resolver_test.go

@@ -184,7 +184,7 @@ func newResolverWithChannel(t *testing.T, pricing []ChannelModelPricing) *ModelP
 			return map[int64]string{groupID: "anthropic"}, nil
 		},
 	}
-	cs := NewChannelService(repo, nil, nil)
+	cs := NewChannelService(repo, nil, nil, nil)
 	bs := newTestBillingServiceForResolver()
 	return NewModelPricingResolver(cs, bs)
 }
@@ -517,7 +517,7 @@ func TestResolve_WithChannelOverride_CacheError(t *testing.T) {
 			return nil, errors.New("database unavailable")
 		},
 	}
-	cs := NewChannelService(repo, nil, nil)
+	cs := NewChannelService(repo, nil, nil, nil)
 	bs := newTestBillingServiceForResolver()
 	r := NewModelPricingResolver(cs, bs)