feat(billing): 统一计价与候选冻结估算

新增 effective-pricing-v2、9 位十进制定点计算、显式免费校验和结构化缺价错误。估价与生产预处理覆盖全部可用候选，并按最大候选费用冻结；规则优先级为平台模型、平台、基准模型。\n\n同步计价契约和 OpenAPI，补充 Token 参数别名、视频五秒向上取整及缺价回归测试。\n\n验证：go test ./...、pnpm openapi、Web 测试与构建通过。

apps/api/internal/httpapi/pricing_handlers_test.go

@@ -0,0 +1,28 @@
+package httpapi
+
+import (
+	"testing"
+
+	"github.com/easyai/easyai-ai-gateway/apps/api/internal/store"
+)
+
+func TestPricingRuleSetRejectsImplicitZeroAndUnsupportedCalculator(t *testing.T) {
+	base := store.PricingRuleSetInput{
+		RuleSetKey: "test", Name: "test",
+		Rules: []store.PricingRuleInput{{
+			ResourceType: "image", Unit: "image", BasePrice: 0,
+			Currency: "resource", CalculatorType: "unit_weight",
+		}},
+	}
+	if validPricingRuleSetInput(base) {
+		t.Fatal("implicit zero price must be rejected")
+	}
+	base.Rules[0].IsFree = true
+	if !validPricingRuleSetInput(base) {
+		t.Fatal("explicit free price should be accepted")
+	}
+	base.Rules[0].CalculatorType = "formula"
+	if validPricingRuleSetInput(base) {
+		t.Fatal("arbitrary formula calculator must be rejected")
+	}
+}