feat: add debug log control and optimize error response and cors log

- add DebugLog global variable controlled by BYTEDANCE_TTS_DEBUG env
- enable debug logs for cors, tts upstream, response parsing etc when debug mode is on
- update tts error response to use standardized json error format
- add dashboard and metrics path to cache-control exempt list

controller/tts.go

@@ -162,7 +162,7 @@ func OpenaiTTSHandler(w http.ResponseWriter, r *http.Request) {
 		metrics.RequestDuration.Observe(duration.Seconds(), telemetry.Labels{"status": finalLabels["status"], "format": clientFormat})
 		log.Printf("警告: TTS 合成失败 - 路径=%s 客户端=%s 文本长度=%d 耗时=%v 错误=%v",
 			r.URL.Path, middleware.GetClientIP(r), len(req.Input), duration, err)
-		http.Error(w, "TTS synthesis failed", http.StatusInternalServerError)
+		middleware.SendJSONError(w, http.StatusInternalServerError, "TTS synthesis failed.", "server_error", "synthesis_failed")
 		return
 	}