fix(billing): 视频时长按实际秒数线性计费

将五秒基础价按 duration / 5 比例结算，保留 provider 返回的小数时长，避免六秒视频被按两个完整单位收费。

影响：所有使用 5s 视频基础价的模型，quantity 与 durationUnitCount 允许小数。新增迁移同步现存规则的旧 ceil 公式元数据。

验证：go vet ./...；pnpm lint；pnpm test；pnpm build；./tests/ci/migrations-test.sh

apps/api/internal/runner/pricing_v2.go

@@ -184,6 +184,22 @@ func multiplyFixedAmountRatio(amount fixedAmount, numerator int, denominator int
 	return fixedAmountFromBigInt(roundBigIntRatio(product, big.NewInt(int64(denominator))))
 }
 
+func multiplyFixedProductRatio(base fixedAmount, integerFactors []int, fixedFactors []fixedAmount, denominator int) (fixedAmount, error) {
+	if denominator == 0 {
+		return 0, fmt.Errorf("division by zero")
+	}
+	product := big.NewInt(int64(base))
+	for _, factor := range integerFactors {
+		product.Mul(product, big.NewInt(int64(factor)))
+	}
+	divisor := big.NewInt(int64(denominator))
+	for _, factor := range fixedFactors {
+		product.Mul(product, big.NewInt(int64(factor)))
+		divisor.Mul(divisor, big.NewInt(fixedScale))
+	}
+	return fixedAmountFromBigInt(roundBigIntRatio(product, divisor))
+}
+
 func fixedAmountFromBigInt(value *big.Int) (fixedAmount, error) {
 	if value == nil || !value.IsInt64() {
 		return 0, errFixedAmountOverflow
@@ -649,7 +665,11 @@ func (s *Service) billingsWithResolvedPricingV2(
 		baseKey = "videoBase"
 		duration, durationSource := billingDurationSeconds(body, response)
 		audioEnabled, audioSource := billingAudioEnabled(body, response)
-		durationUnits := int(math.Max(1, math.Ceil(duration/5)))
+		durationUnits := videoDurationUnits(duration)
+		durationFixed, durationErr := fixedAmountFromAny(duration)
+		if durationErr != nil {
+			return nil, 0, resolvedPricing{}, pricing.calculationError(resource, durationErr)
+		}
 		price, priceErr := pricing.requiredPrice(resource, baseKey, "basePrice")
 		if priceErr != nil {
 			return nil, 0, resolvedPricing{}, priceErr
@@ -670,11 +690,16 @@ func (s *Service) billingsWithResolvedPricingV2(
 		if weightErr != nil {
 			return nil, 0, resolvedPricing{}, weightErr
 		}
-		amount, calculationErr := pricing.calculate(resource, price, []int{count, durationUnits}, resolutionWeight, audioWeight, referenceVideoWeight, voiceWeight, discount)
+		amount, calculationErr := multiplyFixedProductRatio(
+			price,
+			[]int{count},
+			[]fixedAmount{durationFixed, resolutionWeight, audioWeight, referenceVideoWeight, voiceWeight, discount},
+			videoBillingUnitSeconds,
+		)
 		if calculationErr != nil {
-			return nil, 0, resolvedPricing{}, calculationErr
+			return nil, 0, resolvedPricing{}, pricing.calculationError(resource, calculationErr)
 		}
-		item := buildLine(resource, unit, count*durationUnits, amount, map[string]any{
+		item := buildLine(resource, unit, videoDurationQuantity(duration, count), amount, map[string]any{
 			"count": count, "audio": audioEnabled, "audioSource": audioSource,
 			"durationSeconds": duration, "durationSource": durationSource,
 			"durationUnit": "5s", "durationUnitCount": durationUnits,