refactor(tts): 简化火山TTS实现，移除可配置格式采样率等参数

重构了火山TTS的适配器与配置逻辑，移除了格式、采样率、model等可配置项，固定使用wav/24000Hz参数，简化请求流程并删除冗余的格式转换、WAV头拼装逻辑，同时更新文档与环境变量配置。

controller/tts.go

@@ -115,14 +115,8 @@ func OpenaiTTSHandler(w http.ResponseWriter, r *http.Request) {
 		speed = common.MaxSpeed
 	}
 
-	// 将 OpenAI response_format 映射为火山 API 支持的格式
-	var requestFormat string
-	if req.ResponseFormat != "" {
-		requestFormat = volcano.MapOpenAIFormat(req.ResponseFormat)
-	}
-
 	ttsStart := time.Now()
-	result, err := volcano.Synthesis(&setting.TTSConfig, volcanoClient, req.Input, speed, req.Voice, requestFormat)
+	result, err := volcano.Synthesis(&setting.TTSConfig, volcanoClient, req.Input, speed)
 	duration := time.Since(ttsStart)
 
 	if err != nil {
@@ -135,7 +129,7 @@ func OpenaiTTSHandler(w http.ResponseWriter, r *http.Request) {
 
 	service.GlobalStats.AddRequest(true, duration, "")
 
-	w.Header().Set("Content-Type", volcano.FormatContentType(result.Format))
+	w.Header().Set("Content-Type", "audio/wav")
 	w.Header().Set("Content-Length", fmt.Sprintf("%d", len(result.AudioData)))
 	w.Header().Set("X-Request-Id", result.ReqID)
 	w.WriteHeader(http.StatusOK)