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_error_test.go

@@ -0,0 +1,27 @@
+package httpapi
+
+import (
+	"net/http"
+	"testing"
+
+	"github.com/easyai/easyai-ai-gateway/apps/api/internal/runner"
+)
+
+func TestPricingUnavailableUsesStructuredServiceUnavailableError(t *testing.T) {
+	err := &runner.PricingUnavailableError{
+		Reason:       "missing, invalid, or not explicitly free",
+		ResourceType: "image",
+		RuleSetID:    "rule-set-id",
+	}
+	if got := statusFromRunError(err); got != http.StatusServiceUnavailable {
+		t.Fatalf("statusFromRunError()=%d, want %d", got, http.StatusServiceUnavailable)
+	}
+	if got := runErrorCode(err); got != "pricing_unavailable" {
+		t.Fatalf("runErrorCode()=%q, want pricing_unavailable", got)
+	}
+	details := runErrorDetails(err)
+	pricing, _ := details["pricing"].(map[string]any)
+	if pricing["resourceType"] != "image" || pricing["ruleSetId"] != "rule-set-id" {
+		t.Fatalf("unexpected pricing details: %+v", details)
+	}
+}