feat: 实现 Deepgram STT 服务及测试（WebSocket mock，覆盖成功/超时/空音频/多轮 final）

backend/internal/ai/stt/deepgram.go

@@ -95,16 +95,13 @@ func (d *DeepgramService) Recognize(ctx context.Context, audio []byte, opts Opti
 			continue
 		}
 
-		if len(resp.Channel.Alternatives) > 0 {
+		// 只累积 final 结果，跳过中间结果
+		if resp.IsFinal && len(resp.Channel.Alternatives) > 0 {
 			text := strings.TrimSpace(resp.Channel.Alternatives[0].Transcript)
 			if text != "" {
 				transcript.WriteString(text)
 			}
 		}
-
-		if resp.IsFinal {
-			break
-		}
 	}
 
 	return strings.TrimSpace(transcript.String()), nil