fix: STT 服务去除重复默认值，新增 timeout 参数

- Deepgram/MiMo STT 超时从 config 传入
- 去除 model/endpoint 的 fallback 默认值，由 config 层保证

backend/internal/ai/stt/deepgram.go

@@ -18,21 +18,22 @@ type DeepgramService struct {
 	apiKey   string
 	model    string
 	endpoint string
+	timeout  time.Duration
 	logger   *zap.SugaredLogger
 }
 
 // NewDeepgramService 创建 Deepgram STT 服务。
-func NewDeepgramService(apiKey, model, endpoint string, logger *zap.SugaredLogger) *DeepgramService {
-	if model == "" {
-		model = "nova-2"
-	}
-	if endpoint == "" {
-		endpoint = "wss://api.deepgram.com/v1/listen"
+// model、endpoint 由 config 层保证非空，timeoutSec 为 0 时默认 5 秒。
+func NewDeepgramService(apiKey, model, endpoint string, timeoutSec int, logger *zap.SugaredLogger) *DeepgramService {
+	timeout := time.Duration(timeoutSec) * time.Second
+	if timeout <= 0 {
+		timeout = 5 * time.Second
 	}
 	return &DeepgramService{
 		apiKey:   apiKey,
 		model:    model,
 		endpoint: endpoint,
+		timeout:  timeout,
 		logger:   logger,
 	}
 }
@@ -57,8 +58,8 @@ func (d *DeepgramService) Recognize(ctx context.Context, audio []byte, opts Opti
 	// 构建 WebSocket URL，附带查询参数
 	wsURL := d.buildURL(opts)
 
-	// 5 秒总超时
-	ctx, cancel := context.WithTimeout(ctx, 5*time.Second)
+	// 总超时
+	ctx, cancel := context.WithTimeout(ctx, d.timeout)
 	defer cancel()
 
 	// 建立 WebSocket 连接