refactor(backend): 将ApiUser模型重命名为Admin并更新相关引用

backend/init_database.js

@@ -1,5 +1,5 @@
 const { sequelize } = require('./models');
-const { ApiUser, Order } = require('./models');
+const { Admin, Order } = require('./models');
 const bcrypt = require('bcryptjs');
 
 // 演示账号数据
@@ -100,7 +100,7 @@ const initDatabase = async () => {
       const passwordHash = await bcrypt.hash(userData.password, salt);
       
       try {
-        await ApiUser.create({
+        await Admin.create({
           ...userData,
           password_hash: passwordHash
         });