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

common/constants.go

@@ -0,0 +1,19 @@
+package common
+
+import "time"
+
+const (
+	DefaultPort           = "8080"
+	DefaultTimeout        = 30 * time.Second
+	MaxTextLength         = 5000
+	MinSpeed              = 0.25
+	MaxSpeed              = 4.0
+	DefaultSpeed          = 1.0
+	MaxRequestBodySize    = 1024 * 1024
+	RateLimitRequests     = 100
+	RateLimitWindow       = time.Minute
+	MaxResponseTimes      = 100
+	MaxErrors             = 10
+	MaxConcurrentRequests = 10
+	CleanupInterval       = time.Hour
+)