fix: 四个TTS引擎添加友好中文错误提示（欠费/密钥无效/频率限制）

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

backend/app/services/fish_audio_service.py

@@ -43,11 +43,17 @@ def synthesize_fish_audio(text, speed=1.0, pitch=0, volume=1.0, emotion='neutral
         auth=(PROXY_USER, PROXY_PASS), timeout=120
     )
 
+    if response.status_code == 402:
+        raise Exception('Fish Audio 余额不足，请联系管理员充值')
+    if response.status_code == 401:
+        raise Exception('Fish Audio API密钥无效，请联系管理员')
+    if response.status_code == 429:
+        raise Exception('Fish Audio 请求过于频繁，请稍后再试')
     if response.status_code != 200:
-        raise Exception(f'Fish Audio API错误: {response.status_code} - {response.text[:200]}')
+        raise Exception(f'Fish Audio 服务异常({response.status_code})，请稍后再试')
 
     audio_bytes = response.content
     if not audio_bytes:
-        raise Exception('Fish Audio返回空音频')
+        raise Exception('Fish Audio 返回空音频，请稍后再试')
 
     return audio_bytes