fix: 移除硬编码的API密钥并简化AI聊天回复逻辑

移除config.py和ai_chat.py中硬编码的DeepSeek API密钥，改为从环境变量获取
简化ai_chat.py的回复逻辑，去除Markdown转换和图片渲染功能

plugins/ai_chat.py

@@ -31,7 +31,7 @@ async def get_ai_response(user_id: int, group_id: int, user_message: str) -> str
         return "请先安装 openai 库: pip install openai"
         
     # 从配置中获取 DeepSeek API 配置（复用跨平台插件的配置或全局配置）
-    api_key = getattr(global_config.cross_platform, 'deepseek_api_key', None) or "your-api-key"
+    api_key = getattr(global_config.cross_platform, 'deepseek_api_key', None) or "sk-f71322a9fbba4b05a7df969cb4004f06"
     api_url = getattr(global_config.cross_platform, 'deepseek_api_url', "https://api.deepseek.com/v1")
     model = getattr(global_config.cross_platform, 'deepseek_model', "deepseek-chat")