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/skillbundle/content/skills/ai-gateway-ops-management/references/model-billing-configuration-and-estimation.md

@@ -166,16 +166,16 @@ The response has this shape:
       "platformModelId": "<platform-model-id>",
       "resourceType": "video",
       "unit": "5s_video",
-      "quantity": 3,
+      "quantity": 2.4,
       "amount": 12.5,
       "currency": "resource",
       "discountFactor": 0.8,
       "simulated": true,
       "durationSeconds": 12,
-      "durationUnitCount": 3
+      "durationUnitCount": 2.4
     }
   ],
-  "resolver": "effective-pricing-v1",
+  "resolver": "effective-pricing-v2",
   "totalAmount": 12.5,
   "currency": "resource"
 }
@@ -197,7 +197,7 @@ Useful calculation checks:
 text input  = input tokens / 1000 × input price × discount
 text output = output tokens / 1000 × output price × discount
 image       = count × base price × quality/size/resolution weights × discount
-video       = count × ceil(duration seconds / 5) × base price × applicable weights × discount
+video       = count × (duration seconds / 5) × base price × applicable weights × discount
 speech      = Unicode character count × audio price × discount
 ```