refactor: migrate to WeChat cloud development

- 新增 cloud-functions/ 目录（6个云函数：auth/device/subscription/treatment/user/admin）
- 云函数使用 wx-server-sdk + 云数据库（不再依赖 MySQL）
- auth 云函数通过 cloud.getWXContext() 自动获取 openid，无需手动 wx.login
- request.js 改为通过 wx.cloud.callFunction() 调用云函数
- 登录流程简化：点击授权 → 直接调 auth 云函数 → 获取用户信息
- 保留 HTTP 模式切换能力（USE_CLOUD 变量控制）

miniprogram/cloud-functions/auth/index.js

@@ -0,0 +1,51 @@
+const cloud = require('wx-server-sdk')
+cloud.init({ env: cloud.DYNAMIC_CURRENT_ENV })
+const db = cloud.database()
+
+exports.main = async (event, context) => {
+  const wxContext = cloud.getWXContext()
+  const openid = wxContext.OPENID
+
+  if (event.action === 'login') {
+    let userRecord = await db.collection('users').where({ openid }).get()
+
+    if (userRecord.data.length === 0) {
+      const newUser = {
+        openid,
+        nickname: '',
+        avatar_url: '',
+        phone: '',
+        gender: 0,
+        created_at: db.serverDate()
+      }
+      await db.collection('users').add({ data: newUser })
+      userRecord = await db.collection('users').where({ openid }).get()
+    }
+
+    const user = userRecord.data[0]
+    return {
+      code: 0,
+      data: {
+        token: openid + '_' + Date.now(),
+        user_id: user._id,
+        user_info: {
+          user_id: user._id,
+          nickname: user.nickname || '用户' + openid.slice(-4),
+          avatar_url: user.avatar_url || '',
+          phone: user.phone || '',
+          gender: user.gender || 0
+        }
+      }
+    }
+  }
+
+  if (event.action === 'refresh') {
+    const userRecord = await db.collection('users').where({ openid }).get()
+    if (userRecord.data.length === 0) {
+      return { code: 1001, message: '用户不存在' }
+    }
+    return { code: 0, data: { token: openid + '_' + Date.now() } }
+  }
+
+  return { code: -1, message: '未知操作' }
+}