feat: 接入豆包 Seed-ICL 2.0 语音合成引擎

- 新增 doubao_service.py 调用火山引擎 TTS API
- 路由加 doubao 引擎分支
- 前端引擎下拉菜单加豆包选项

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/app.js

@@ -17,6 +17,11 @@ const ENGINE_CONFIG = {
         name: 'CosyVoice',
         emotionMode: 'global',
         color: '#ff6b6b'
+    },
+    doubao: {
+        name: '豆包',
+        emotionMode: 'global',
+        color: '#7c5cfc'
     }
 };