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

middleware/performance.go

@@ -0,0 +1,71 @@
+package middleware
+
+import (
+	"fmt"
+	"net/http"
+	"strings"
+
+	"github.com/QuantumNous/new-api/common"
+	"github.com/QuantumNous/new-api/relaykit/types"
+	"github.com/gin-gonic/gin"
+)
+
+// SystemPerformanceCheck 检查系统性能中间件
+func SystemPerformanceCheck() gin.HandlerFunc {
+	return func(c *gin.Context) {
+		// 仅检查 Relay 接口 (/v1, /v1beta 等)
+		// 这里简单判断路径前缀，可以根据实际路由调整
+		path := c.Request.URL.Path
+		if strings.HasPrefix(path, "/v1/messages") {
+			if err := checkSystemPerformance(); err != nil {
+				c.JSON(err.StatusCode, gin.H{
+					"error": err.ToClaudeError(),
+				})
+				c.Abort()
+				return
+			}
+		} else {
+			if err := checkSystemPerformance(); err != nil {
+				c.JSON(err.StatusCode, gin.H{
+					"error": err.ToOpenAIError(),
+				})
+				c.Abort()
+				return
+			}
+		}
+		c.Next()
+	}
+}
+
+// checkSystemPerformance 检查系统性能是否超过阈值
+func checkSystemPerformance() *types.NewAPIError {
+	config := common.GetPerformanceMonitorConfig()
+	if !config.Enabled {
+		return nil
+	}
+
+	status := common.GetSystemStatus()
+
+	// 检查 CPU
+	if config.CPUThreshold > 0 && int(status.CPUUsage) > config.CPUThreshold {
+		return types.NewErrorWithStatusCode(
+			fmt.Errorf("system cpu overloaded (current: %.1f%%, threshold: %d%%)", status.CPUUsage, config.CPUThreshold),
+			"system_cpu_overloaded", http.StatusServiceUnavailable)
+	}
+
+	// 检查内存
+	if config.MemoryThreshold > 0 && int(status.MemoryUsage) > config.MemoryThreshold {
+		return types.NewErrorWithStatusCode(
+			fmt.Errorf("system memory overloaded (current: %.1f%%, threshold: %d%%)", status.MemoryUsage, config.MemoryThreshold),
+			"system_memory_overloaded", http.StatusServiceUnavailable)
+	}
+
+	// 检查磁盘
+	if config.DiskThreshold > 0 && int(status.DiskUsage) > config.DiskThreshold {
+		return types.NewErrorWithStatusCode(
+			fmt.Errorf("system disk overloaded (current: %.1f%%, threshold: %d%%)", status.DiskUsage, config.DiskThreshold),
+			"system_disk_overloaded", http.StatusServiceUnavailable)
+	}
+
+	return nil
+}