refactor: 重构项目架构，拆分代码到模块化目录

将单文件tts_server.go重构为模块化项目结构，拆分出common、dto、middleware、router、controller、service、adapter、setting等目录，优化代码组织提升可维护性

main.go

@@ -0,0 +1,82 @@
+package main
+
+import (
+	"context"
+	"log"
+	"net/http"
+	"os"
+	"os/signal"
+	"syscall"
+	"time"
+
+	"github.com/volcano-tts/tts-api/common"
+	"github.com/volcano-tts/tts-api/controller"
+	"github.com/volcano-tts/tts-api/middleware"
+	"github.com/volcano-tts/tts-api/router"
+	"github.com/volcano-tts/tts-api/service"
+	"github.com/volcano-tts/tts-api/setting"
+)
+
+func main() {
+	log.SetFlags(log.LstdFlags | log.Lshortfile)
+	log.SetPrefix("[TTS-Server] ")
+
+	middleware.InitAPIKeys()
+	middleware.InitCORSConfig()
+	middleware.InitRateLimiter()
+	setting.CheckStaticFiles()
+	service.InitStats()
+	controller.InitController()
+
+	setting.TTSConfigErr = setting.InitTTSConfig()
+	if setting.TTSConfigErr != nil {
+		log.Printf("警告: 配置初始化失败: %v", setting.TTSConfigErr)
+		log.Printf("服务将继续运行，但TTS功能不可用，请检查环境变量配置")
+	} else {
+		log.Printf("配置初始化成功")
+	}
+
+	controller.SetStartTime(time.Now())
+
+	r := router.Setup()
+
+	port := os.Getenv("PORT")
+	if port == "" {
+		port = common.DefaultPort
+	}
+
+	server := &http.Server{
+		Addr:         ":" + port,
+		Handler:      r,
+		ReadTimeout:  30 * time.Second,
+		WriteTimeout: 120 * time.Second,
+		IdleTimeout:  60 * time.Second,
+	}
+
+	quit := make(chan os.Signal, 1)
+	signal.Notify(quit, syscall.SIGINT, syscall.SIGTERM)
+
+	go func() {
+		log.Printf("Starting ByteDance TTS to OpenAI API Adapter Server")
+		log.Printf("Listening on port: %s", port)
+		log.Printf("OpenAI TTS endpoint: http://localhost:%s/v1/audio/speech", port)
+		log.Printf("Health check: http://localhost:%s/health", port)
+		log.Printf("Using ByteDance v3 API")
+
+		if err := server.ListenAndServe(); err != nil && err != http.ErrServerClosed {
+			log.Fatalf("Server failed to start: %v", err)
+		}
+	}()
+
+	<-quit
+	log.Println("Shutting down server...")
+
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+
+	if err := server.Shutdown(ctx); err != nil {
+		log.Printf("Server forced to shutdown: %v", err)
+	} else {
+		log.Println("Server exited gracefully")
+	}
+}