fix: MiMo ASR 认证头改为 Authorization: Bearer

MiMo API 使用标准 Bearer Token 认证，而非自定义 api-key 头。

backend/internal/ai/stt/mimo.go

@@ -134,7 +134,7 @@ func (m *MiMoService) Recognize(ctx context.Context, audio []byte, opts Options)
 		return "", fmt.Errorf("stt: create request: %w", err)
 	}
 	req.Header.Set("Content-Type", "application/json")
-	req.Header.Set("api-key", m.apiKey)
+	req.Header.Set("Authorization", "Bearer "+m.apiKey)
 
 	resp, err := http.DefaultClient.Do(req)
 	if err != nil {