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.wxss

@@ -118,3 +118,13 @@
 .btn-primary-gold::after {
   border: none;
 }
+
+.plan-disabled {
+  opacity: 0.5;
+  pointer-events: none;
+}
+
+.plan-tag-disabled {
+  background: #ccc;
+  color: #fff;
+}