feat: show trial plan with used/disabled state on subscribe page

- Add GET /api/v1/subscription/plans public endpoint for pricing
- Subscription API now returns trial_used field
- Subscribe-plans page shows trial card, greyed out with "已使用" tag
  when trial has been used
- Trial activation calls dedicated trial API, not purchase
- Prices fetched from server settings, fallback to defaults

server/src/routes/subscription.js

@@ -27,13 +27,15 @@ router.get('/subscription/plans', wrap(async (req, res) => {
 
 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 }))
+  const trialUsed = await subscriptionDao.findTrial(req.user.user_id)
+  if (!sub) return res.json(ok({ status: 'inactive', plan: 'none', remaining_days: 0, trial_used: !!trialUsed }))
   res.json(ok({
     status: sub.remaining_days > 0 ? 'active' : 'expired',
     plan: sub.plan,
     start_time: sub.start_time,
     expire_time: sub.expire_time,
-    remaining_days: sub.remaining_days
+    remaining_days: sub.remaining_days,
+    trial_used: !!trialUsed
   }))
 }))