refactor: 实现 ConversionEngine 协议转换引擎，替代旧 protocol 包

引入 Canonical Model 和 ProtocolAdapter 架构，支持 OpenAI/Anthropic 协议间
无缝转换，统一 ProxyHandler 替代分散的 OpenAI/Anthropic Handler，简化
ProviderClient 为协议无关的 HTTP 发送器，Provider 新增 protocol 字段。

backend/cmd/server/main.go

@@ -20,6 +20,9 @@ import (
 	"gorm.io/gorm/logger"
 
 	"nex/backend/internal/config"
+	"nex/backend/internal/conversion"
+	"nex/backend/internal/conversion/anthropic"
+	"nex/backend/internal/conversion/openai"
 	"nex/backend/internal/handler"
 	"nex/backend/internal/handler/middleware"
 	"nex/backend/internal/provider"
@@ -70,30 +73,37 @@ func main() {
 	routingService := service.NewRoutingService(modelRepo, providerRepo)
 	statsService := service.NewStatsService(statsRepo)
 
-	// 6. 初始化 provider client
+	// 6. 创建 ConversionEngine
+	registry := conversion.NewMemoryRegistry()
+	if err := registry.Register(openai.NewAdapter()); err != nil {
+		zapLogger.Fatal("注册 OpenAI 适配器失败", zap.String("error", err.Error()))
+	}
+	if err := registry.Register(anthropic.NewAdapter()); err != nil {
+		zapLogger.Fatal("注册 Anthropic 适配器失败", zap.String("error", err.Error()))
+	}
+	engine := conversion.NewConversionEngine(registry)
+
+	// 7. 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)
+	// 8. 初始化 handler 层
+	proxyHandler := handler.NewProxyHandler(engine, providerClient, routingService, providerService, statsService)
 	providerHandler := handler.NewProviderHandler(providerService)
 	modelHandler := handler.NewModelHandler(modelService)
 	statsHandler := handler.NewStatsHandler(statsService)
 
-	// 8. 创建 Gin 引擎
+	// 9. 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, openaiHandler, anthropicHandler, providerHandler, modelHandler, statsHandler)
+	setupRoutes(r, proxyHandler, providerHandler, modelHandler, statsHandler)
 
-	// 9. 启动服务器
+	// 10. 启动服务器
 	srv := &http.Server{
 		Addr:         formatAddr(cfg.Server.Port),
 		Handler:      r,
@@ -108,7 +118,6 @@ func main() {
 		}
 	}()
 
-	// 等待中断信号
 	quit := make(chan os.Signal, 1)
 	signal.Notify(quit, syscall.SIGINT, syscall.SIGTERM)
 	<-quit
@@ -137,12 +146,10 @@ func initDatabase(cfg *config.Config) (*gorm.DB, error) {
 		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
@@ -151,14 +158,12 @@ func initDatabase(cfg *config.Config) (*gorm.DB, error) {
 	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 {
@@ -178,18 +183,14 @@ func runMigrations(db *gorm.DB) error {
 	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"
 }
 
@@ -205,12 +206,9 @@ 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
-	r.POST("/v1/chat/completions", openaiHandler.HandleChatCompletions)
-
-	// Anthropic 协议代理
-	r.POST("/v1/messages", anthropicHandler.HandleMessages)
+func setupRoutes(r *gin.Engine, proxyHandler *handler.ProxyHandler, providerHandler *handler.ProviderHandler, modelHandler *handler.ModelHandler, statsHandler *handler.StatsHandler) {
+	// 统一代理入口: /{protocol}/v1/{path}
+	r.Any("/:protocol/v1/*path", proxyHandler.HandleProxy)
 
 	// 供应商管理 API
 	providers := r.Group("/api/providers")