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/handlers.go

@@ -892,6 +892,7 @@ func (s *Server) deleteAPIKey(w http.ResponseWriter, r *http.Request) {
 // @Failure 403 {object} ErrorEnvelope
 // @Failure 404 {object} ErrorEnvelope
 // @Failure 429 {object} ErrorEnvelope
+// @Failure 503 {object} ErrorEnvelope
 // @Failure 500 {object} ErrorEnvelope
 // @Router /api/v1/pricing/estimate [post]
 func (s *Server) estimatePricing(w http.ResponseWriter, r *http.Request) {
@@ -916,6 +917,10 @@ func (s *Server) estimatePricing(w http.ResponseWriter, r *http.Request) {
 	}
 	estimate, err := s.runner.Estimate(r.Context(), kind, model, body, user)
 	if err != nil {
+		if runner.IsPricingUnavailable(err) {
+			writeErrorWithDetails(w, http.StatusServiceUnavailable, runErrorMessage(err), runErrorDetails(err), "pricing_unavailable")
+			return
+		}
 		if errors.Is(err, store.ErrNoModelCandidate) {
 			writeErrorWithDetails(w, statusFromRunError(err), runErrorMessage(err), runErrorDetails(err), store.ModelCandidateErrorCode(err))
 			return
@@ -1383,6 +1388,8 @@ func scopeForTaskKind(kind string) string {
 
 func statusFromRunError(err error) int {
 	switch {
+	case runner.IsPricingUnavailable(err):
+		return http.StatusServiceUnavailable
 	case clients.ErrorCode(err) == "invalid_previous_response_id" || clients.ErrorCode(err) == "response_chain_too_deep" || clients.ErrorCode(err) == "unsupported_model_protocol" || clients.ErrorCode(err) == "unsupported_response_tool" || clients.ErrorCode(err) == "unsupported_response_parameter":
 		return http.StatusBadRequest
 	case clients.ErrorCode(err) == "response_chain_unavailable":
@@ -1411,6 +1418,9 @@ func statusFromRunError(err error) int {
 }
 
 func runErrorCode(err error) string {
+	if runner.IsPricingUnavailable(err) {
+		return "pricing_unavailable"
+	}
 	if errors.Is(err, store.ErrNoModelCandidate) {
 		return store.ModelCandidateErrorCode(err)
 	}
@@ -1428,6 +1438,9 @@ func runErrorMessage(err error) string {
 }
 
 func runErrorDetails(err error) map[string]any {
+	if detail := runner.PricingUnavailableDetails(err); len(detail) > 0 {
+		return map[string]any{"pricing": detail}
+	}
 	if detail := rateLimitErrorDetail(err); len(detail) > 0 {
 		return map[string]any{"rateLimit": detail}
 	}