fix(errors): 区分平台限流并保留上游状态码

原因：公开错误层将平台并发限流误标为上游限流，并把多种上游 4xx 统一压成 400，影响定位和客户端处理。

影响：新增公开错误 source，平台限流使用 gateway_rate_limited，上游请求按安全分类返回对应状态；数据库与管理端继续保留原始错误码、消息和状态用于审计。

验证：Go 全量测试、pnpm test、pnpm lint、pnpm build、pnpm openapi、gofmt 和 diff 检查均通过。

apps/api/internal/publicerror/metrics.go

@@ -15,6 +15,7 @@ var observed = struct {
 	upstreamInvalidResponse       atomic.Uint64
 	upstreamAuthFailed            atomic.Uint64
 	upstreamRequestRejected       atomic.Uint64
+	gatewayRateLimited            atomic.Uint64
 	storageWriteFailed            atomic.Uint64
 	storageReadFailed             atomic.Uint64
 	resultExpired                 atomic.Uint64
@@ -39,6 +40,10 @@ func Observe(value Error) {
 		observed.upstreamAuthFailed.Add(1)
 	case "upstream_request_rejected":
 		observed.upstreamRequestRejected.Add(1)
+	case "upstream_invalid_request", "upstream_not_found", "upstream_method_not_allowed", "upstream_conflict", "upstream_payload_too_large", "upstream_unsupported_media_type", "upstream_unprocessable_request":
+		observed.upstreamRequestRejected.Add(1)
+	case "gateway_rate_limited":
+		observed.gatewayRateLimited.Add(1)
 	case "storage_write_failed":
 		observed.storageWriteFailed.Add(1)
 	case "storage_read_failed":
@@ -63,6 +68,7 @@ func MetricSnapshot() []MetricCount {
 		{Code: "upstream_invalid_response", Count: observed.upstreamInvalidResponse.Load()},
 		{Code: "upstream_auth_failed", Count: observed.upstreamAuthFailed.Load()},
 		{Code: "upstream_request_rejected", Count: observed.upstreamRequestRejected.Load()},
+		{Code: "gateway_rate_limited", Count: observed.gatewayRateLimited.Load()},
 		{Code: "storage_write_failed", Count: observed.storageWriteFailed.Load()},
 		{Code: "storage_read_failed", Count: observed.storageReadFailed.Load()},
 		{Code: "result_expired", Count: observed.resultExpired.Load()},