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.go

@@ -10,6 +10,8 @@ import (
 	"github.com/easyai/easyai-ai-gateway/apps/api/internal/store"
 )
 
+const videoBillingUnitSeconds = 5
+
 type EstimateResult struct {
 	Items              []any   `json:"items"`
 	Resolver           string  `json:"resolver"`
@@ -135,7 +137,7 @@ func (s *Service) billings(ctx context.Context, user *auth.User, kind string, bo
 		baseKey = "videoBase"
 		duration, durationSource := billingDurationSeconds(body, response)
 		audioEnabled, audioSource := billingAudioEnabled(body, response)
-		durationUnits := math.Max(1, math.Ceil(duration/5))
+		durationUnits := videoDurationUnits(duration)
 		amount := float64(count) *
 			durationUnits *
 			resourcePrice(config, resource, baseKey, "basePrice") *
@@ -144,7 +146,7 @@ func (s *Service) billings(ctx context.Context, user *auth.User, kind string, bo
 			resourceWeight(config, resource, "referenceVideoWeights", boolWeightKey(requestHasReferenceVideo(body))) *
 			resourceWeight(config, resource, "voiceSpecifiedWeights", boolWeightKey(requestHasVoiceID(body, audioEnabled))) *
 			discount
-		return []any{billingLineWithDetails(candidate, resource, unit, count*int(durationUnits), roundPrice(amount), discount, simulated, map[string]any{
+		return []any{billingLineWithDetails(candidate, resource, unit, videoDurationQuantity(duration, count), roundPrice(amount), discount, simulated, map[string]any{
 			"count":             count,
 			"audio":             audioEnabled,
 			"audioSource":       audioSource,
@@ -416,6 +418,16 @@ func weightValueAliases(key string, name string) []string {
 	}
 }
 
+func videoDurationUnits(durationSeconds float64) float64 {
+	return videoDurationQuantity(durationSeconds, 1)
+}
+
+func videoDurationQuantity(durationSeconds float64, count int) float64 {
+	const durationPrecision = 1_000_000_000
+	scaledDuration := math.Round(durationSeconds * durationPrecision)
+	return scaledDuration * float64(count) / (durationPrecision * videoBillingUnitSeconds)
+}
+
 func requestOutputCount(body map[string]any) int {
 	for _, key := range []string{"n", "count", "batch_size", "batchSize"} {
 		if value := int(math.Ceil(floatFromAny(body[key]))); value > 0 {
@@ -476,11 +488,7 @@ func generatedVideoDurationSeconds(result map[string]any) (float64, bool) {
 		if duration <= 0 {
 			continue
 		}
-		rounded := math.Round(duration)
-		if rounded <= 0 {
-			rounded = 1
-		}
-		return rounded, true
+		return duration, true
 	}
 	return 0, false
 }