feat(billing): 网关计费迁移到 CalculateCostUnified + 模型限制错误统一

- GatewayService/OpenAIGatewayService 注入 ModelPricingResolver
- RecordUsage 从旧路径迁移到 CalculateCostUnified（支持 per_request/image 模式）
- 无渠道时自动回退旧路径，保持原有行为
- 长上下文双倍计费仅在无渠道定价时生效
- CostBreakdown 新增 BillingMode 字段，使用日志记录实际计费模式
- 模型限制错误改为与"无可用账号"相同的 503 响应

backend/internal/service/openai_gateway_service.go

@@ -322,6 +322,7 @@ type OpenAIGatewayService struct {
 	openAITokenProvider   *OpenAITokenProvider
 	toolCorrector         *CodexToolCorrector
 	openaiWSResolver      OpenAIWSProtocolResolver
+	resolver              *ModelPricingResolver
 
 	openaiWSPoolOnce              sync.Once
 	openaiWSStateStoreOnce        sync.Once
@@ -357,6 +358,7 @@ func NewOpenAIGatewayService(
 	httpUpstream HTTPUpstream,
 	deferredService *DeferredService,
 	openAITokenProvider *OpenAITokenProvider,
+	resolver *ModelPricingResolver,
 ) *OpenAIGatewayService {
 	svc := &OpenAIGatewayService{
 		accountRepo:         accountRepo,
@@ -384,6 +386,7 @@ func NewOpenAIGatewayService(
 		openAITokenProvider:   openAITokenProvider,
 		toolCorrector:         NewCodexToolCorrector(),
 		openaiWSResolver:      NewOpenAIWSProtocolResolver(cfg),
+		resolver:              resolver,
 		responseHeaderFilter:  compileResponseHeaderFilter(cfg),
 		codexSnapshotThrottle: newAccountWriteThrottle(openAICodexSnapshotPersistMinInterval),
 	}
@@ -4152,12 +4155,28 @@ func (s *OpenAIGatewayService) RecordUsage(ctx context.Context, input *OpenAIRec
 		multiplier = resolver.Resolve(ctx, user.ID, *apiKey.GroupID, apiKey.Group.RateMultiplier)
 	}
 
+	var cost *CostBreakdown
+	var err error
 	billingModel := forwardResultBillingModel(result.Model, result.UpstreamModel)
 	serviceTier := ""
 	if result.ServiceTier != nil {
 		serviceTier = strings.TrimSpace(*result.ServiceTier)
 	}
-	cost, err := s.billingService.CalculateCostWithServiceTier(billingModel, tokens, multiplier, serviceTier)
+	if s.resolver != nil && apiKey.Group != nil {
+		gid := apiKey.Group.ID
+		cost, err = s.billingService.CalculateCostUnified(CostInput{
+			Ctx:            ctx,
+			Model:          billingModel,
+			GroupID:        &gid,
+			Tokens:         tokens,
+			RequestCount:   1,
+			RateMultiplier: multiplier,
+			ServiceTier:    serviceTier,
+			Resolver:       s.resolver,
+		})
+	} else {
+		cost, err = s.billingService.CalculateCostWithServiceTier(billingModel, tokens, multiplier, serviceTier)
+	}
 	if err != nil {
 		cost = &CostBreakdown{ActualCost: 0}
 	}
@@ -4204,8 +4223,11 @@ func (s *OpenAIGatewayService) RecordUsage(ctx context.Context, input *OpenAIRec
 		FirstTokenMs:          result.FirstTokenMs,
 		CreatedAt:             time.Now(),
 	}
-	// 设置计费模式（OpenAI 网关都是 token 计费）
-	{
+	// 设置计费模式
+	if cost != nil && cost.BillingMode != "" {
+		billingMode := cost.BillingMode
+		usageLog.BillingMode = &billingMode
+	} else {
 		billingMode := "token"
 		usageLog.BillingMode = &billingMode
 	}