feat(config): add WECHAT_MP credentials for mini wechat login.

Co-authored-by: Cursor <cursoragent@cursor.com>

backend/.env.example

@@ -76,3 +76,7 @@ DEFAULT_TENANT_NO=shanxia2.0
 # WXPAY_NOTIFY_URL=https://www.chengyantek.cn/api/payment/notify
 # WXPAY_REFUND_NOTIFY_URL=https://www.chengyantek.cn/api/payment/refund-notify
 # WXPAY_SANDBOX=false
+
+# 门户小程序登录（AppID 与 miniprogram/manifest.json mp-weixin.appid 一致）
+# WECHAT_MP_APPID=wx7db1ddb2a90c9425
+# WECHAT_MP_SECRET=