feat(channels): 渠道未填价时按 LiteLLM 默认价展示

「可用渠道」展示链路有两个未覆盖场景导致用户看到"未配置定价":

1. admin 在 UI 里建了 ModelPricing 条目但没填任何价格 (常见于
   per_request / image 模式只填了 tier_label 没填单价): 原 fallback
   只检查 Pricing == nil, 这种空条目会跳过 LiteLLM 兜底。
2. LiteLLM 把图片模型标记 mode=image_generation, 但合成器固定按
   token 模式合成, 把 OutputCostPerImage / 图片 token 价丢到错误字段。

改动 (仅 backend/internal/service/channel_available.go):
- 新增 pricingNeedsFallback: 价格字段全空 (含 intervals 全空) 视为
  未配置, 触发 LiteLLM 兜底。
- synthesizePricingFromLiteLLM 加 existing 参数: 优先尊重渠道已选
  BillingMode (per_request / image 也按此模式合成), 没选才看 LiteLLM
  mode, 仍未命中默认 token。
- image / per_request 分支用 OutputCostPerImage 填 PerRequestPrice,
  OutputCostPerImageToken 填 ImageOutputPrice, 让 gpt-image / dall-e
  系列展示出参考价。

仅影响展示链路, 真实计费走 BillingService / ModelPricingResolver
完全不受影响。新增 8 个单元测试覆盖 pricingNeedsFallback 各分支、
合成器三种模式选择、空条目兜底与既有价格保护。

backend/internal/service/channel_available.go

@@ -103,7 +103,11 @@ func (s *ChannelService) ListAvailable(ctx context.Context) ([]AvailableChannel,
 }
 
 // fillGlobalPricingFallback 对未命中渠道定价的支持模型，从全局 LiteLLM 数据合成一份
-// 展示用定价（按 token 计费）。仅用于「可用渠道」展示，不影响真实计费链路。
+// 展示用定价。仅用于「可用渠道」展示，不影响真实计费链路。
+//
+// 触发条件：
+//  1. Pricing == nil（渠道完全没声明该模型的定价条目）
+//  2. Pricing 非 nil 但所有价格字段为空（admin UI 建了条目但没填价格）
 //
 // 当 s.pricingService 为 nil（测试场景），跳过回落。
 func (s *ChannelService) fillGlobalPricingFallback(models []SupportedModel) {
@@ -111,28 +115,72 @@ func (s *ChannelService) fillGlobalPricingFallback(models []SupportedModel) {
 		return
 	}
 	for i := range models {
-		if models[i].Pricing != nil {
+		if !pricingNeedsFallback(models[i].Pricing) {
 			continue
 		}
 		lp := s.pricingService.GetModelPricing(models[i].Name)
 		if lp == nil {
 			continue
 		}
-		models[i].Pricing = synthesizePricingFromLiteLLM(lp)
+		models[i].Pricing = synthesizePricingFromLiteLLM(lp, models[i].Pricing)
 	}
 }
 
+// pricingNeedsFallback 判定一个 ChannelModelPricing 是否需要走全局回落。
+// 价格全部缺失（无 flat 字段且无任何带价 interval）即视为未配置。
+func pricingNeedsFallback(p *ChannelModelPricing) bool {
+	if p == nil {
+		return true
+	}
+	if p.InputPrice != nil || p.OutputPrice != nil ||
+		p.CacheWritePrice != nil || p.CacheReadPrice != nil ||
+		p.ImageOutputPrice != nil || p.PerRequestPrice != nil {
+		return false
+	}
+	for _, iv := range p.Intervals {
+		if iv.InputPrice != nil || iv.OutputPrice != nil ||
+			iv.CacheWritePrice != nil || iv.CacheReadPrice != nil ||
+			iv.PerRequestPrice != nil {
+			return false
+		}
+	}
+	return true
+}
+
 // synthesizePricingFromLiteLLM 把 LiteLLM 的定价数据转成 ChannelModelPricing 形态，
-// 仅用于展示。BillingMode 固定为 token；图片场景的 OutputCostPerImageToken 也归到
+// 仅用于展示。
-// ImageOutputPrice 字段（与渠道侧"图片输出按 token 计价"语义一致）。
+//
+// 计费模式优先级：
+//  1. 渠道已选 BillingMode（admin 在 UI 里选了 image / per_request 但没填价的场景，
+//     按选定模式合成对应字段）
+//  2. LiteLLM mode="image_generation" → image
+//  3. 默认 token
 //
 // LiteLLM 中字段 0 视为未配置，不带入展示。
-func synthesizePricingFromLiteLLM(lp *LiteLLMModelPricing) *ChannelModelPricing {
+func synthesizePricingFromLiteLLM(lp *LiteLLMModelPricing, existing *ChannelModelPricing) *ChannelModelPricing {
 	if lp == nil {
-		return nil
+		return existing
+	}
+
+	mode := BillingModeToken
+	switch {
+	case existing != nil && existing.BillingMode != "":
+		mode = existing.BillingMode
+	case lp.Mode == "image_generation":
+		mode = BillingModeImage
+	}
+
+	if mode == BillingModeImage || mode == BillingModePerRequest {
+		return &ChannelModelPricing{
+			BillingMode:      mode,
+			PerRequestPrice:  nonZeroPtr(lp.OutputCostPerImage),
+			ImageOutputPrice: nonZeroPtr(lp.OutputCostPerImageToken),
+			InputPrice:       nonZeroPtr(lp.InputCostPerToken),
+			OutputPrice:      nonZeroPtr(lp.OutputCostPerToken),
+		}
 	}
 	return &ChannelModelPricing{
-		BillingMode:      BillingModeToken,
+		BillingMode:      mode,
 		InputPrice:       nonZeroPtr(lp.InputCostPerToken),
 		OutputPrice:      nonZeroPtr(lp.OutputCostPerToken),
 		CacheWritePrice:  nonZeroPtr(lp.CacheCreationInputTokenCost),

backend/internal/service/channel_available_test.go

@@ -175,3 +175,137 @@ func TestListAvailable_DefaultsEmptyBillingModelSource(t *testing.T) {
 	require.Equal(t, BillingModelSourceChannelMapped, byName["empty"])
 	require.Equal(t, BillingModelSourceUpstream, byName["explicit"])
 }
+
+func TestPricingNeedsFallback(t *testing.T) {
+	tests := []struct {
+		name string
+		in   *ChannelModelPricing
+		want bool
+	}{
+		{"nil", nil, true},
+		{"empty struct", &ChannelModelPricing{BillingMode: BillingModeToken}, true},
+		{"all-empty intervals", &ChannelModelPricing{
+			BillingMode: BillingModeImage,
+			Intervals:   []PricingInterval{{TierLabel: "1K"}, {TierLabel: "2K"}},
+		}, true},
+		{"flat input set", &ChannelModelPricing{InputPrice: testPtrFloat64(3e-6)}, false},
+		{"flat per_request set", &ChannelModelPricing{PerRequestPrice: testPtrFloat64(0.04)}, false},
+		{"interval with price", &ChannelModelPricing{
+			Intervals: []PricingInterval{{TierLabel: "1K", PerRequestPrice: testPtrFloat64(0.04)}},
+		}, false},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			require.Equal(t, tt.want, pricingNeedsFallback(tt.in))
+		})
+	}
+}
+
+func TestSynthesizePricingFromLiteLLM_TokenMode(t *testing.T) {
+	lp := &LiteLLMModelPricing{
+		Mode:                        "chat",
+		InputCostPerToken:           3e-6,
+		OutputCostPerToken:          1.5e-5,
+		CacheCreationInputTokenCost: 3.75e-6,
+		CacheReadInputTokenCost:     3e-7,
+	}
+	got := synthesizePricingFromLiteLLM(lp, nil)
+	require.NotNil(t, got)
+	require.Equal(t, BillingModeToken, got.BillingMode)
+	require.NotNil(t, got.InputPrice)
+	require.InDelta(t, 3e-6, *got.InputPrice, 1e-12)
+	require.NotNil(t, got.CacheReadPrice)
+}
+
+func TestSynthesizePricingFromLiteLLM_ImageGenerationMode(t *testing.T) {
+	// LiteLLM mode=image_generation 且渠道未声明模式时，按 image 合成。
+	lp := &LiteLLMModelPricing{
+		Mode:                    "image_generation",
+		OutputCostPerImageToken: 4e-5,
+	}
+	got := synthesizePricingFromLiteLLM(lp, nil)
+	require.NotNil(t, got)
+	require.Equal(t, BillingModeImage, got.BillingMode)
+	require.Nil(t, got.PerRequestPrice)
+	require.NotNil(t, got.ImageOutputPrice)
+}
+
+func TestSynthesizePricingFromLiteLLM_RespectsExistingChannelMode(t *testing.T) {
+	// admin UI 选了 per_request 但没填价：LiteLLM 数据按 per_request 合成,
+	// 即便 LiteLLM 标的是 chat 模式也尊重渠道选择。
+	lp := &LiteLLMModelPricing{
+		Mode:               "chat",
+		InputCostPerToken:  5e-6,
+		OutputCostPerImage: 0.04,
+	}
+	existing := &ChannelModelPricing{BillingMode: BillingModePerRequest}
+	got := synthesizePricingFromLiteLLM(lp, existing)
+	require.NotNil(t, got)
+	require.Equal(t, BillingModePerRequest, got.BillingMode)
+	require.NotNil(t, got.PerRequestPrice)
+	require.InDelta(t, 0.04, *got.PerRequestPrice, 1e-12)
+}
+
+func TestFillGlobalPricingFallback_NilPricing(t *testing.T) {
+	pricingSvc := newStubPricingServiceFromMap(map[string]*LiteLLMModelPricing{
+		"claude-opus-4-5": {Mode: "chat", InputCostPerToken: 5e-6},
+	})
+	svc := &ChannelService{pricingService: pricingSvc}
+
+	models := []SupportedModel{
+		{Name: "claude-opus-4-5", Platform: "anthropic"},
+	}
+	svc.fillGlobalPricingFallback(models)
+	require.NotNil(t, models[0].Pricing)
+	require.NotNil(t, models[0].Pricing.InputPrice)
+	require.InDelta(t, 5e-6, *models[0].Pricing.InputPrice, 1e-12)
+}
+
+func TestFillGlobalPricingFallback_EmptyPricingFillsFromLiteLLM(t *testing.T) {
+	// 核心场景：admin UI 建了 pricing 条目（image 模式）但没填价，应走 LiteLLM 兜底。
+	pricingSvc := newStubPricingServiceFromMap(map[string]*LiteLLMModelPricing{
+		"gpt-image-1": {
+			Mode:                    "image_generation",
+			OutputCostPerImageToken: 4e-5,
+		},
+	})
+	svc := &ChannelService{pricingService: pricingSvc}
+
+	models := []SupportedModel{
+		{
+			Name:     "gpt-image-1",
+			Platform: "openai",
+			Pricing: &ChannelModelPricing{
+				BillingMode: BillingModeImage,
+				Intervals:   []PricingInterval{{TierLabel: "1K"}, {TierLabel: "2K"}},
+			},
+		},
+	}
+	svc.fillGlobalPricingFallback(models)
+	require.NotNil(t, models[0].Pricing)
+	require.Equal(t, BillingModeImage, models[0].Pricing.BillingMode)
+	require.NotNil(t, models[0].Pricing.ImageOutputPrice)
+	require.InDelta(t, 4e-5, *models[0].Pricing.ImageOutputPrice, 1e-12)
+}
+
+func TestFillGlobalPricingFallback_KeepsExistingPrice(t *testing.T) {
+	// 渠道已经填了价格的条目不应被回落覆盖。
+	pricingSvc := newStubPricingServiceFromMap(map[string]*LiteLLMModelPricing{
+		"served-model": {Mode: "chat", InputCostPerToken: 1e-6},
+	})
+	svc := &ChannelService{pricingService: pricingSvc}
+
+	existing := &ChannelModelPricing{
+		BillingMode: BillingModeToken,
+		InputPrice:  testPtrFloat64(9e-9),
+	}
+	models := []SupportedModel{
+		{Name: "served-model", Platform: "anthropic", Pricing: existing},
+	}
+	svc.fillGlobalPricingFallback(models)
+	require.Same(t, existing, models[0].Pricing)
+}
+
+func newStubPricingServiceFromMap(data map[string]*LiteLLMModelPricing) *PricingService {
+	return &PricingService{pricingData: data}
+}