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/minimax_service.py

@@ -38,8 +38,14 @@ def synthesize_minimax(text, speed=1.0, pitch=0, volume=1.0, emotion='neutral'):
     
     response = requests.post(url, json=payload, headers=headers, timeout=60)
     
+    if response.status_code == 402:
+        raise Exception('MiniMax 余额不足，请联系管理员充值')
+    if response.status_code == 401 or response.status_code == 403:
+        raise Exception('MiniMax API密钥无效，请联系管理员')
+    if response.status_code == 429:
+        raise Exception('MiniMax 请求过于频繁，请稍后再试')
     if response.status_code != 200:
-        raise Exception(f'MiniMax API错误: {response.status_code} - {response.text}')
+        raise Exception(f'MiniMax 服务异常({response.status_code})，请稍后再试')
     
     result = response.json()