feat: 接入 Fish Audio TTS 引擎

- 新增 fish_audio_service.py，使用 REST API 调用
- voice_id: 34e841f0f4ff44aca6e396debf2776d4（蓝皓音色克隆）
- 支持语速/音量控制，情绪通过内嵌标签实现
- 前端引擎下拉菜单新增 Fish Audio 选项

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/routes/tts_synthesize.py

@@ -4,6 +4,7 @@ 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.services.fish_audio_service import synthesize_fish_audio
 from app.routes.auth import login_required
 from app.db import get_db
 
@@ -43,6 +44,11 @@ def synthesize():
                 text=text, speed=speed, pitch=pitch,
                 volume=volume, emotion=emotion
             )
+        elif engine == 'fish_audio':
+            audio_data = synthesize_fish_audio(
+                text=text, speed=speed, pitch=pitch,
+                volume=volume, emotion=emotion
+            )
         else:
             audio_data = synthesize_minimax(
                 text=text, speed=speed, pitch=pitch,