fix(billing): enforce one effective subscription per user

frontend/src/pages/admin/AdminBillingPage.vue

@@ -212,7 +212,7 @@ onMounted(async () => {
         >
           {{ subBusy ? '提交中…' : '立即开通' }}
         </button>
-        <span class="text-xs text-slate-500">会取消该用户当前有效订阅，并按月数顺延。</span>
+        <span class="text-xs text-slate-500">会替换当前本地套餐；存在未取消 Stripe 订阅时将拒绝操作。</span>
       </div>
 
       <div v-if="subMessage" class="mt-3 rounded-lg border border-emerald-200 bg-emerald-50 p-3 text-sm text-emerald-900">