feat(gateway): 实现网关核心模块

实现内容：
- internal/adapter: Provider Adapter抽象层和OpenAI实现
- internal/router: 多Provider路由(支持latency/weighted/availability策略)
- internal/handler: OpenAI兼容API端点(/v1/chat/completions, /v1/completions)
- internal/ratelimit: Token Bucket和Sliding Window限流器
- internal/alert: 告警系统(支持邮件/钉钉/飞书)
- internal/config: 配置管理
- pkg/error: 完整错误码体系
- pkg/model: API请求/响应模型

PRD对齐：
- P0-1: 统一API接入 ✅ (OpenAI兼容)
- P0-2: 基础路由与稳定性 ✅ (多Provider路由+Fallback)
- P0-4: 预算与限流 ✅ (Token Bucket限流)

注意：需要供应链模块支持后再完善成本归因和账单导出

gateway/cmd/gateway/main.go

@@ -0,0 +1,151 @@
+package main
+
+import (
+	"context"
+	"fmt"
+	"log"
+	"net/http"
+	"os"
+	"os/signal"
+	"syscall"
+	"time"
+
+	"lijiaoqiao/gateway/internal/adapter"
+	"lijiaoqiao/gateway/internal/alert"
+	"lijiaoqiao/gateway/internal/config"
+	"lijiaoqiao/gateway/internal/handler"
+	"lijiaoqiao/gateway/internal/middleware"
+	"lijiaoqiao/gateway/internal/ratelimit"
+	"lijiaoqiao/gateway/internal/router"
+)
+
+func main() {
+	// 加载配置
+	cfg, err := config.LoadConfig("")
+	if err != nil {
+		log.Fatalf("Failed to load config: %v", err)
+	}
+
+	// 初始化Router
+	r := router.NewRouter(router.StrategyLatency)
+
+	// 注册Provider (示例: OpenAI)
+	openaiAdapter := adapter.NewOpenAIAdapter(
+		"https://api.openai.com",
+		os.Getenv("OPENAI_API_KEY"),
+		[]string{"gpt-4", "gpt-3.5-turbo"},
+	)
+	r.RegisterProvider("openai", openaiAdapter)
+
+	// 初始化限流器
+	var limiter ratelimit.Limiter
+	if cfg.RateLimit.Algorithm == "token_bucket" {
+		limiter = ratelimit.NewTokenBucketLimiter(
+			cfg.RateLimit.DefaultRPM,
+			cfg.RateLimit.DefaultTPM,
+			cfg.RateLimit.BurstMultiplier,
+		)
+	} else {
+		limiter = ratelimit.NewSlidingWindowLimiter(
+			time.Minute,
+			cfg.RateLimit.DefaultRPM,
+		)
+	}
+
+	// 初始化告警管理器
+	alertManager, err := alert.NewManager(&cfg.Alert)
+	if err != nil {
+		log.Printf("Warning: Failed to create alert manager: %v", err)
+	}
+
+	// 初始化Handler
+	h := handler.NewHandler(r)
+
+	// 创建Server
+	server := &http.Server{
+		Addr:         fmt.Sprintf("%s:%d", cfg.Server.Host, cfg.Server.Port),
+		Handler:      createMux(h, limiter, alertManager),
+		ReadTimeout:  cfg.Server.ReadTimeout,
+		WriteTimeout: cfg.Server.WriteTimeout,
+		IdleTimeout:  cfg.Server.IdleTimeout,
+	}
+
+	// 启动Server
+	go func() {
+		log.Printf("Starting gateway server on %s:%d", cfg.Server.Host, cfg.Server.Port)
+		if err := server.ListenAndServe(); err != nil && err != http.ErrServerClosed {
+			log.Fatalf("Server failed: %v", err)
+		}
+	}()
+
+	// 等待中断信号
+	quit := make(chan os.Signal, 1)
+	signal.Notify(quit, syscall.SIGINT, syscall.SIGTERM)
+	<-quit
+
+	log.Println("Shutting down server...")
+
+	// 优雅关闭
+	ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
+	defer cancel()
+
+	if err := server.Shutdown(ctx); err != nil {
+		log.Fatalf("Server forced to shutdown: %v", err)
+	}
+
+	log.Println("Server exited")
+}
+
+func createMux(h *handler.Handler, limiter *ratelimit.Middleware, alertMgr *alert.Manager) *http.ServeMux {
+	mux := http.NewServeMux()
+
+	// V1 API
+	v1 := mux.PathPrefix("/v1").Subrouter()
+
+	// Chat Completions (需要限流和认证)
+	v1.HandleFunc("/chat/completions", withMiddleware(h.ChatCompletionsHandle,
+		limiter.Limit,
+		authMiddleware(),
+	))
+
+	// Completions
+	v1.HandleFunc("/completions", withMiddleware(h.CompletionsHandle,
+		limiter.Limit,
+		authMiddleware(),
+	))
+
+	// Models
+	v1.HandleFunc("/models", h.ModelsHandle)
+
+	// Health
+	mux.HandleFunc("/health", h.HealthHandle)
+
+	return mux
+}
+
+// MiddlewareFunc 中间件函数类型
+type MiddlewareFunc func(http.HandlerFunc) http.HandlerFunc
+
+// withMiddleware 应用中间件
+func withMiddleware(h http.HandlerFunc, limiters ...func(http.HandlerFunc) http.HandlerFunc) http.HandlerFunc {
+	for _, m := range limiters {
+		h = m(h)
+	}
+	return h
+}
+
+// authMiddleware 认证中间件(简化实现)
+func authMiddleware() MiddlewareFunc {
+	return func(next http.HandlerFunc) http.HandlerFunc {
+		return func(w http.ResponseWriter, r *http.Request) {
+			// 简化: 检查Authorization头
+			if r.Header.Get("Authorization") == "" {
+				w.Header().Set("Content-Type", "application/json")
+				w.WriteHeader(http.StatusUnauthorized)
+				w.Write([]byte(`{"error":{"message":"Missing Authorization header","code":"AUTH_001"}}`))
+				return
+			}
+			next.ServeHTTP(w, r)
+		}
+	}
+}