feat: 算力引擎 Go 后端核心（new-api）

- 统一 OpenAI 兼容 /v1 中继 + 渠道/额度/令牌/流水
- 各领域包：relay 模型网关、model 数据层、controller 管理 API

pkg/billingexpr/round.go

@@ -0,0 +1,19 @@
+package billingexpr
+
+import "github.com/QuantumNous/new-api/common"
+
+// QuotaRound converts a float64 quota value to int using half-away-from-zero
+// rounding with int32 saturation. Every tiered billing path (pre-consume,
+// settlement, breakdown validation, log fields) MUST use this function to
+// avoid +-1 discrepancies.
+//
+// It delegates to common.QuotaRound so all quota rounding/conversion shares
+// one saturation + logging policy (see common/quota_math.go).
+func QuotaRound(f float64) int {
+	return common.QuotaRound(f)
+}
+
+// QuotaRoundStrict rejects an unrepresentable pre-consume estimate.
+func QuotaRoundStrict(f float64) (int, error) {
+	return common.QuotaRoundStrict(f)
+}