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>

backend/app/routes/tts_synthesize.py

@@ -3,6 +3,7 @@ import base64
 import os
 from app.services.minimax_service import synthesize_minimax
 from app.services.cosyvoice_service import synthesize_cosyvoice
+from app.services.doubao_service import synthesize_doubao
 from app.routes.auth import login_required
 from app.db import get_db
 
@@ -37,6 +38,11 @@ def synthesize():
                 text=text, speed=speed, pitch=pitch,
                 volume=volume, instruction=instruction
             )
+        elif engine == 'doubao':
+            audio_data = synthesize_doubao(
+                text=text, speed=speed, pitch=pitch,
+                volume=volume, emotion=emotion
+            )
         else:
             audio_data = synthesize_minimax(
                 text=text, speed=speed, pitch=pitch,