feat: 日志级别优化（Phase 7）

- nodes_stt.go: STT 识别开始降为 Debug
- nodes_history.go: 历史组装完成降为 Debug
- nodes_tts.go: TTS 流中断降为 Debug
- 保持关键里程碑为 Info：query completed、tts synthesis started/completed
- 中间步骤详情降为 Debug，减少生产环境日志噪音

backend/internal/eino/nodes_history.go

@@ -121,7 +121,7 @@ func NewHistoryLambda(
 			})
 		}
 
-		log.Infow("history assembled",
+		log.Debugw("history assembled",
 			"message_count", len(messages),
 			"has_image", len(imageData) > 0,
 			"scenario", scenario)

backend/internal/eino/nodes_stt.go

@@ -75,7 +75,7 @@ func NewSTTLambda(sttService stt.Service) *compose.Lambda {
 			return STTOutput{}, fmt.Errorf("stt: no audio data provided")
 		}
 
-		log.Infow("stt recognition started", "audio_bytes", len(input.AudioData))
+		log.Debugw("stt recognition started", "audio_bytes", len(input.AudioData))
 
 		// 调用 STT 服务
 		text, err := sttService.Recognize(ctx, input.AudioData, stt.Options{

backend/internal/eino/nodes_tts.go

@@ -87,7 +87,7 @@ func NewTTSLambda(ttsService tts.Service, ttsVoice string, ttsSpeed float64, tts
 			for chunk := range ttsStream {
 				select {
 				case <-ctx.Done():
-					log.Infow("TTS 流被中断")
+					log.Debugw("tts stream interrupted")
 					sw.Send(struct{}{}, ctx.Err())
 					return
 				default: