fix(env): 修复其他提供商没有办法使用识别问题

backend/.env.example

@@ -11,13 +11,13 @@ DATA_DIR=data
 # 后端专用
 
 # AI 相关配置
-OPENAI_API_KEY = ""
-OPENAI_API_BASE_URL = ""
-OPENAI_MODEL = ""
-DEEP_SEEK_API_KEY = ""
-DEEP_SEEK_API_BASE_URL = ""
-DEEP_SEEK_MODEL = ""
-QWEN_API_KEY = ""
-QWEN_API_BASE_URL = ""
-QWEN_MODEL = ""
-
+OPENAI_API_KEY=
+OPENAI_API_BASE_URL=
+OPENAI_MODEL=
+DEEP_SEEK_API_KEY=
+DEEP_SEEK_API_BASE_URL=
+DEEP_SEEK_MODEL
+QWEN_API_KEY=
+QWEN_API_BASE_URL=
+QWEN_MODEL=
+MODEl_PROVIDER= #如果不是openai 请修改 deepseek/qwen