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/services/fish_audio_service.py

@@ -0,0 +1,46 @@
+import os
+import requests
+
+
+def synthesize_fish_audio(text, speed=1.0, pitch=0, volume=1.0, emotion='neutral'):
+    api_key = os.getenv('FISH_AUDIO_API_KEY', '')
+    if not api_key:
+        raise ValueError('FISH_AUDIO_API_KEY未设置')
+
+    voice_id = os.getenv('FISH_AUDIO_VOICE_ID', '34e841f0f4ff44aca6e396debf2776d4')
+
+    emotion_tags = {
+        'happy': '[happy]',
+        'sad': '[sad]',
+        'angry': '[angry]',
+        'surprised': '[excited]',
+        'fearful': '[nervous]',
+    }
+    tag = emotion_tags.get(emotion, '')
+    if tag:
+        text = f"{tag}{text}"
+
+    url = 'https://api.fish.audio/v1/tts'
+    headers = {
+        'Authorization': f'Bearer {api_key}',
+        'Content-Type': 'application/json'
+    }
+
+    payload = {
+        'text': text,
+        'reference_id': voice_id,
+        'format': 'mp3',
+        'speed': max(0.5, min(2.0, speed)),
+        'volume': max(-20, min(20, (volume - 1.0) * 20)),
+    }
+
+    response = requests.post(url, json=payload, headers=headers, timeout=60, stream=True)
+
+    if response.status_code != 200:
+        raise Exception(f'Fish Audio API错误: {response.status_code} - {response.text[:200]}')
+
+    audio_bytes = response.content
+    if not audio_bytes:
+        raise Exception('Fish Audio返回空音频')
+
+    return audio_bytes