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

miniprogram/pages/subscribe-plans/subscribe-plans.wxml

@@ -29,8 +29,8 @@
     </view>
 
     <view class="plans">
-      <view class="plan {{selected === item.key ? 'selected' : ''}}" wx:for="{{plans}}" wx:key="key" bindtap="onSelect" data-plan="{{item.key}}">
-        <view class="plan-tag {{selected === item.key ? 'plan-tag-active' : ''}}" wx:if="{{item.tag}}">{{item.tag}}</view>
+      <view class="plan {{selected === item.key ? 'selected' : ''}} {{item.disabled ? 'plan-disabled' : ''}}" wx:for="{{plans}}" wx:key="key" bindtap="{{item.disabled ? '' : 'onSelect'}}" data-plan="{{item.key}}">
+        <view class="plan-tag {{item.disabled ? 'plan-tag-disabled' : (selected === item.key ? 'plan-tag-active' : '')}}" wx:if="{{item.tag}}">{{item.tag}}</view>
         <view class="plan-price">{{item.priceLabel}}</view>
         <view class="plan-name">{{item.name}}</view>
         <view class="plan-save">{{item.desc}}</view>