fix(billing): 按次计费回退到默认 PerRequestPrice

ResolvedPricing 新增 DefaultPerRequestPrice，当无层级匹配时使用渠道的默认按次价格

backend/internal/service/billing_service.go

@@ -542,6 +542,11 @@ func (s *BillingService) calculatePerRequestCost(resolved *ResolvedPricing, inpu
 		unitPrice = input.Resolver.GetRequestTierPriceByContext(resolved, totalContext)
 	}
 
+	// 回退到默认按次价格
+	if unitPrice == 0 {
+		unitPrice = resolved.DefaultPerRequestPrice
+	}
+
 	totalCost := unitPrice * float64(count)
 	actualCost := totalCost * input.RateMultiplier
 

backend/internal/service/model_pricing_resolver.go

@@ -19,6 +19,9 @@ type ResolvedPricing struct {
 	// 按次/图片模式：分层定价
 	RequestTiers []PricingInterval
 
+	// 按次/图片模式：默认价格（未命中层级时使用）
+	DefaultPerRequestPrice float64
+
 	// 来源标识
 	Source string // "channel", "litellm", "fallback"
 
@@ -136,6 +139,9 @@ func (r *ModelPricingResolver) applyTokenOverrides(chPricing *ChannelModelPricin
 // applyRequestTierOverrides 应用按次/图片模式的渠道覆盖
 func (r *ModelPricingResolver) applyRequestTierOverrides(chPricing *ChannelModelPricing, resolved *ResolvedPricing) {
 	resolved.RequestTiers = chPricing.Intervals
+	if chPricing.PerRequestPrice != nil {
+		resolved.DefaultPerRequestPrice = *chPricing.PerRequestPrice
+	}
 }
 
 // GetIntervalPricing 根据 context token 数获取区间定价。