feat: 实现分层架构，包含 domain、service、repository 和 pkg 层

- 新增 domain 层：model、provider、route、stats 实体
- 新增 service 层：models、providers、routing、stats 业务逻辑
- 新增 repository 层：models、providers、stats 数据访问
- 新增 pkg 工具包：errors、logger、validator
- 新增中间件：CORS、logging、recovery、request ID
- 新增数据库迁移：初始 schema 和索引
- 新增单元测试和集成测试
- 新增规范文档：config-management、database-migration、error-handling、layered-architecture、middleware-system、request-validation、structured-logging、test-coverage
- 移除 config 子包和 model_router（已迁移至分层架构）

backend/cmd/server/main.go

@@ -2,88 +2,217 @@ package main
 
 import (
 	"context"
+	"fmt"
 	"log"
 	"net/http"
 	"os"
 	"os/signal"
+	"path/filepath"
+	"runtime"
 	"syscall"
 	"time"
 
 	"github.com/gin-gonic/gin"
+	"github.com/pressly/goose/v3"
+	"go.uber.org/zap"
+	"gorm.io/driver/sqlite"
+	"gorm.io/gorm"
+	"gorm.io/gorm/logger"
 
 	"nex/backend/internal/config"
 	"nex/backend/internal/handler"
+	"nex/backend/internal/handler/middleware"
+	"nex/backend/internal/provider"
+	"nex/backend/internal/repository"
+	"nex/backend/internal/service"
+	pkgLogger "nex/backend/pkg/logger"
 )
 
 func main() {
-	// 初始化数据库
-	if err := config.InitDB(); err != nil {
-		log.Fatalf("初始化数据库失败: %v", err)
+	// 1. 加载配置
+	cfg, err := config.LoadConfig()
+	if err != nil {
+		log.Fatalf("加载配置失败: %v", err)
+	}
+	if err := cfg.Validate(); err != nil {
+		log.Fatalf("配置验证失败: %v", err)
 	}
-	defer config.CloseDB()
 
-	// 创建 Gin 引擎
-	gin.SetMode(gin.ReleaseMode)
-	r := gin.Default()
-
-	// 配置 CORS
-	r.Use(func(c *gin.Context) {
-		c.Writer.Header().Set("Access-Control-Allow-Origin", "*")
-		c.Writer.Header().Set("Access-Control-Allow-Methods", "GET, POST, PUT, DELETE, OPTIONS")
-		c.Writer.Header().Set("Access-Control-Allow-Headers", "Content-Type, Authorization")
-		if c.Request.Method == "OPTIONS" {
-			c.AbortWithStatus(204)
-			return
-		}
-		c.Next()
+	// 2. 初始化日志
+	zapLogger, err := pkgLogger.New(pkgLogger.Config{
+		Level:      cfg.Log.Level,
+		Path:       cfg.Log.Path,
+		MaxSize:    cfg.Log.MaxSize,
+		MaxBackups: cfg.Log.MaxBackups,
+		MaxAge:     cfg.Log.MaxAge,
+		Compress:   cfg.Log.Compress,
 	})
+	if err != nil {
+		log.Fatalf("初始化日志失败: %v", err)
+	}
+	defer zapLogger.Sync()
+
+	// 3. 初始化数据库
+	db, err := initDatabase(cfg)
+	if err != nil {
+		zapLogger.Fatal("初始化数据库失败", zap.String("error", err.Error()))
+	}
+	defer closeDB(db)
+
+	// 4. 初始化 repository 层
+	providerRepo := repository.NewProviderRepository(db)
+	modelRepo := repository.NewModelRepository(db)
+	statsRepo := repository.NewStatsRepository(db)
+
+	// 5. 初始化 service 层
+	providerService := service.NewProviderService(providerRepo)
+	modelService := service.NewModelService(modelRepo, providerRepo)
+	routingService := service.NewRoutingService(modelRepo, providerRepo)
+	statsService := service.NewStatsService(statsRepo)
+
+	// 6. 初始化 provider client
+	providerClient := provider.NewClient()
+
+	// 7. 初始化 handler 层
+	openaiHandler := handler.NewOpenAIHandler(providerClient, routingService, statsService)
+	anthropicHandler := handler.NewAnthropicHandler(providerClient, routingService, statsService)
+	providerHandler := handler.NewProviderHandler(providerService)
+	modelHandler := handler.NewModelHandler(modelService)
+	statsHandler := handler.NewStatsHandler(statsService)
+
+	// 8. 创建 Gin 引擎
+	gin.SetMode(gin.ReleaseMode)
+	r := gin.New()
+
+	// 注册中间件（按正确顺序）
+	r.Use(middleware.RequestID())
+	r.Use(middleware.Recovery(zapLogger))
+	r.Use(middleware.Logging(zapLogger))
+	r.Use(middleware.CORS())
 
 	// 注册路由
-	setupRoutes(r)
+	setupRoutes(r, openaiHandler, anthropicHandler, providerHandler, modelHandler, statsHandler)
 
-	// 创建 HTTP 服务器
+	// 9. 启动服务器
 	srv := &http.Server{
-		Addr:    ":9826",
-		Handler: r,
+		Addr:         formatAddr(cfg.Server.Port),
+		Handler:      r,
+		ReadTimeout:  cfg.Server.ReadTimeout,
+		WriteTimeout: cfg.Server.WriteTimeout,
 	}
 
-	// 启动服务器（在 goroutine 中）
 	go func() {
-		log.Printf("AI Gateway 启动在端口 9826")
+		zapLogger.Info("AI Gateway 启动", zap.String("addr", srv.Addr))
 		if err := srv.ListenAndServe(); err != nil && err != http.ErrServerClosed {
-			log.Fatalf("服务器启动失败: %v", err)
+			zapLogger.Fatal("服务器启动失败", zap.String("error", err.Error()))
 		}
 	}()
 
-	// 等待中断信号以优雅关闭服务器
+	// 等待中断信号
 	quit := make(chan os.Signal, 1)
 	signal.Notify(quit, syscall.SIGINT, syscall.SIGTERM)
 	<-quit
-	log.Println("正在关闭服务器...")
 
-	// 给服务器 5 秒时间完成当前请求
+	zapLogger.Info("正在关闭服务器...")
+
 	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
 	defer cancel()
 
 	if err := srv.Shutdown(ctx); err != nil {
-		log.Fatal("服务器强制关闭:", err)
+		zapLogger.Fatal("服务器强制关闭", zap.String("error", err.Error()))
 	}
 
-	log.Println("服务器已关闭")
+	zapLogger.Info("服务器已关闭")
 }
 
-// setupRoutes 配置路由
-func setupRoutes(r *gin.Engine) {
+func initDatabase(cfg *config.Config) (*gorm.DB, error) {
+	db, err := gorm.Open(sqlite.Open(cfg.Database.Path), &gorm.Config{
+		Logger: logger.Default.LogMode(logger.Info),
+	})
+	if err != nil {
+		return nil, err
+	}
+
+	if err := db.Exec("PRAGMA journal_mode=WAL").Error; err != nil {
+		log.Printf("警告: 启用 WAL 模式失败: %v", err)
+	}
+
+	// 运行数据库迁移
+	if err := runMigrations(db); err != nil {
+		return nil, fmt.Errorf("数据库迁移失败: %w", err)
+	}
+
+	// 配置连接池
+	sqlDB, err := db.DB()
+	if err != nil {
+		return nil, err
+	}
+	sqlDB.SetMaxIdleConns(cfg.Database.MaxIdleConns)
+	sqlDB.SetMaxOpenConns(cfg.Database.MaxOpenConns)
+	sqlDB.SetConnMaxLifetime(cfg.Database.ConnMaxLifetime)
+
+	// 记录连接池状态
+	log.Printf("数据库连接池配置: MaxIdle=%d, MaxOpen=%d, MaxLifetime=%v",
+		cfg.Database.MaxIdleConns, cfg.Database.MaxOpenConns, cfg.Database.ConnMaxLifetime)
+
+	return db, nil
+}
+
+// runMigrations 使用 goose 执行数据库迁移
+func runMigrations(db *gorm.DB) error {
+	sqlDB, err := db.DB()
+	if err != nil {
+		return err
+	}
+
+	migrationsDir := getMigrationsDir()
+	if _, err := os.Stat(migrationsDir); os.IsNotExist(err) {
+		return fmt.Errorf("迁移目录不存在: %s", migrationsDir)
+	}
+
+	goose.SetDialect("sqlite3")
+	if err := goose.Up(sqlDB, migrationsDir); err != nil {
+		return err
+	}
+
+	return nil
+}
+
+// getMigrationsDir 获取迁移文件目录路径
+func getMigrationsDir() string {
+	// 从可执行文件位置推断迁移目录
+	_, filename, _, ok := runtime.Caller(0)
+	if ok {
+		// cmd/server/main.go → backend/ → backend/migrations/
+		dir := filepath.Join(filepath.Dir(filename), "..", "..", "migrations")
+		if abs, err := filepath.Abs(dir); err == nil {
+			return abs
+		}
+	}
+	// 回退到相对路径
+	return "./migrations"
+}
+
+func closeDB(db *gorm.DB) {
+	sqlDB, err := db.DB()
+	if err != nil {
+		return
+	}
+	sqlDB.Close()
+}
+
+func formatAddr(port int) string {
+	return fmt.Sprintf(":%d", port)
+}
+
+func setupRoutes(r *gin.Engine, openaiHandler *handler.OpenAIHandler, anthropicHandler *handler.AnthropicHandler, providerHandler *handler.ProviderHandler, modelHandler *handler.ModelHandler, statsHandler *handler.StatsHandler) {
 	// OpenAI 协议代理
-	openaiHandler := handler.NewOpenAIHandler()
 	r.POST("/v1/chat/completions", openaiHandler.HandleChatCompletions)
 
 	// Anthropic 协议代理
-	anthropicHandler := handler.NewAnthropicHandler()
 	r.POST("/v1/messages", anthropicHandler.HandleMessages)
 
 	// 供应商管理 API
-	providerHandler := handler.NewProviderHandler()
 	providers := r.Group("/api/providers")
 	{
 		providers.GET("", providerHandler.ListProviders)
@@ -94,7 +223,6 @@ func setupRoutes(r *gin.Engine) {
 	}
 
 	// 模型管理 API
-	modelHandler := handler.NewModelHandler()
 	models := r.Group("/api/models")
 	{
 		models.GET("", modelHandler.ListModels)
@@ -105,7 +233,6 @@ func setupRoutes(r *gin.Engine) {
 	}
 
 	// 统计查询 API
-	statsHandler := handler.NewStatsHandler()
 	stats := r.Group("/api/stats")
 	{
 		stats.GET("", statsHandler.GetStats)