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/web/src/pages/admin/PricingRuleVisualEditor.tsx

@@ -56,7 +56,7 @@ const modeDefinitions: ModeDefinition[] = [
     formula: '扣费 = 基础单价 × 生成时长单位 × 数量 × 分辨率、音频、参考视频、音色等计费参数。',
     match: (rule) => rule.resourceType === 'video',
     templates: (currency) => [
-      createRule('video', '视频', 'video', '5s', 100, currency, 'duration_weight', 'count * ceil(duration_seconds / 5) * base_price * resolution_factor * audio_factor * reference_video_factor * voice_specified_factor', {
+      createRule('video', '视频', 'video', '5s', 100, currency, 'duration_weight', 'count * (duration_seconds / 5) * base_price * resolution_factor * audio_factor * reference_video_factor * voice_specified_factor', {
         resolutionWeights: { '480p': 0.75, '720p': 1, '1080p': 1.5, '2160p': 2 },
         audioWeights: { true: 2, false: 1 },
         referenceVideoWeights: { true: 1.5, false: 1 },