refactor: unify interval filtering and eliminate redundant Resolve calls

- applyRequestTierOverrides now uses filterValidIntervals consistently
  with applyTokenOverrides (per_request/image modes were not filtering)
- CostInput accepts optional pre-resolved pricing via Resolved field,
  eliminating duplicate Resolver.Resolve() calls in gateway billing paths

backend/internal/service/billing_service.go

@@ -422,6 +422,7 @@ type CostInput struct {
 	RateMultiplier float64
 	ServiceTier    string                // "priority","flex","" 等
 	Resolver       *ModelPricingResolver // 定价解析器
+	Resolved       *ResolvedPricing      // 可选：预解析的定价结果（避免重复 Resolve 调用）
 }
 
 // CalculateCostUnified 统一计费入口，支持三种计费模式。
@@ -432,10 +433,14 @@ func (s *BillingService) CalculateCostUnified(input CostInput) (*CostBreakdown,
 		return s.calculateCostInternal(input.Model, input.Tokens, input.RateMultiplier, input.ServiceTier, nil)
 	}
 
-	resolved := input.Resolver.Resolve(input.Ctx, PricingInput{
+	// 优先使用预解析结果，避免重复 Resolve 调用
+	resolved := input.Resolved
+	if resolved == nil {
+		resolved = input.Resolver.Resolve(input.Ctx, PricingInput{
 			Model:   input.Model,
 			GroupID: input.GroupID,
 		})
+	}
 
 	if input.RateMultiplier <= 0 {
 		input.RateMultiplier = 1.0

backend/internal/service/gateway_service.go

@@ -8007,7 +8007,7 @@ func (s *GatewayService) calculateImageCost(
 	billingModel string,
 	multiplier float64,
 ) *CostBreakdown {
-	if s.resolveChannelPricing(ctx, billingModel, apiKey) != nil {
+	if resolved := s.resolveChannelPricing(ctx, billingModel, apiKey); resolved != nil {
 		tokens := UsageTokens{
 			InputTokens:       result.Usage.InputTokens,
 			OutputTokens:      result.Usage.OutputTokens,
@@ -8022,6 +8022,7 @@ func (s *GatewayService) calculateImageCost(
 			RequestCount:   1,
 			RateMultiplier: multiplier,
 			Resolver:       s.resolver,
+			Resolved:       resolved,
 		})
 		if err != nil {
 			logger.LegacyPrintf("service.gateway", "Calculate image token cost failed: %v", err)
@@ -8064,7 +8065,7 @@ func (s *GatewayService) calculateTokenCost(
 	var err error
 
 	// 优先尝试渠道定价 → CalculateCostUnified
-	if s.resolveChannelPricing(ctx, billingModel, apiKey) != nil {
+	if resolved := s.resolveChannelPricing(ctx, billingModel, apiKey); resolved != nil {
 		gid := apiKey.Group.ID
 		cost, err = s.billingService.CalculateCostUnified(CostInput{
 			Ctx:            ctx,
@@ -8074,6 +8075,7 @@ func (s *GatewayService) calculateTokenCost(
 			RequestCount:   1,
 			RateMultiplier: multiplier,
 			Resolver:       s.resolver,
+			Resolved:       resolved,
 		})
 	} else if opts.LongContextThreshold > 0 {
 		// 长上下文双倍计费（如 Gemini 200K 阈值）

backend/internal/service/model_pricing_resolver.go

@@ -151,7 +151,7 @@ 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
+	resolved.RequestTiers = filterValidIntervals(chPricing.Intervals)
 	if chPricing.PerRequestPrice != nil {
 		resolved.DefaultPerRequestPrice = *chPricing.PerRequestPrice
 	}