feat(client): 添加 Node.js 客户端及示例应用

- 新增 nodejs-client 模块，提供 ConfigClient 配置拉取、定时同步、变更监听
- 使用 Node.js 内置 crypto 模块实现 AES 解密，零外部依赖
- 新增 nodejs-client-demo Express 示例应用，端口 8083
- 更新 README.md，补充 Python 和 Node.js 客户端使用说明
- 修复 fetchAllConfigs API 响应格式判断逻辑

nodejs-client/lib/crypto.js

@@ -0,0 +1,24 @@
+import crypto from 'node:crypto';
+
+const CRYPT_PREFIX = '{Cipher}';
+
+export function decryptConfig(secret, data) {
+  if (!data.startsWith(CRYPT_PREFIX)) {
+    return data;
+  }
+  const encrypted = data.slice(CRYPT_PREFIX.length);
+  return decryptWithAES(secret, encrypted);
+}
+
+export function decryptWithAES(key, data) {
+  const md5Hash = crypto.createHash('md5').update(key).digest('hex');
+  const aesKey = md5Hash.slice(16);
+  const cipherText = Buffer.from(data, 'base64');
+  const iv = Buffer.alloc(16, 0);
+  const decipher = crypto.createDecipheriv('aes-128-cbc', aesKey, iv);
+  decipher.setAutoPadding(false);
+  let plainText = Buffer.concat([decipher.update(cipherText), decipher.final()]);
+  const padding = plainText[plainText.length - 1];
+  plainText = plainText.slice(0, plainText.length - padding);
+  return plainText.toString('utf8');
+}