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/subscription/index.js

@@ -0,0 +1,58 @@
+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
+  const { action } = event
+
+  if (action === 'status') {
+    const subs = await db.collection('subscriptions').where({ openid, status: 'active' }).get()
+    if (subs.data.length === 0) {
+      return { code: 0, data: { plan_type: 'none', status: 'inactive', remaining_days: 0 } }
+    }
+    const sub = subs.data[0]
+    const now = new Date()
+    const end = new Date(sub.end_time)
+    const remain = Math.max(0, Math.ceil((end - now) / (24 * 3600 * 1000)))
+    return {
+      code: 0,
+      data: {
+        plan_type: sub.plan_type,
+        status: remain > 0 ? 'active' : 'expired',
+        remaining_days: remain,
+        start_time: sub.start_time,
+        end_time: sub.end_time
+      }
+    }
+  }
+
+  if (action === 'purchase') {
+    const { plan_type } = event.data
+    const price = plan_type === 'monthly' ? 99 : 899
+    const days = plan_type === 'monthly' ? 30 : 365
+
+    await db.collection('subscriptions').where({ openid, status: 'active' }).update({ data: { status: 'expired' } })
+
+    await db.collection('subscriptions').add({
+      data: {
+        openid,
+        plan_type,
+        status: 'active',
+        start_time: db.serverDate(),
+        end_time: new Date(Date.now() + days * 24 * 3600 * 1000),
+        remaining_days: days,
+        price
+      }
+    })
+
+    return { code: 0, data: { order_id: 'ORD' + Date.now(), plan_type, status: 'paid' } }
+  }
+
+  if (action === 'verify') {
+    return { code: 0, data: { valid: true } }
+  }
+
+  return { code: -1, message: '未知操作' }
+}