feat: fetch subscription prices from server settings

- Add GET /api/v1/subscription/plans public endpoint (no auth needed)
  that reads prices from system_settings table
- Subscribe-plans page now loads prices from server on show
- Falls back to hardcoded defaults if API fails
- Add api.getPlans() to miniprogram API module

server/src/routes/subscription.js

@@ -13,6 +13,18 @@ const PLANS = {
   yearly: { amount: 899, days: 365 }
 }
 
+router.get('/subscription/plans', wrap(async (req, res) => {
+  const settingsDao = require('../dao/settings.dao')
+  const settings = await settingsDao.getAll()
+  res.json(ok({
+    plans: [
+      { key: 'monthly', name: '月卡', price: Number(settings.monthly_price) || PLANS.monthly.amount, days: 30 },
+      { key: 'yearly', name: '年卡', price: Number(settings.yearly_price) || PLANS.yearly.amount, days: 365 },
+      { key: 'trial', name: '试用', price: 0, days: Number(settings.trial_days) || 7 }
+    ]
+  }))
+}))
+
 router.get('/subscription', requireUser, wrap(async (req, res) => {
   const sub = await subscriptionDao.findActive(req.user.user_id)
   if (!sub) return res.json(ok({ status: 'inactive', plan: 'none', remaining_days: 0 }))