永久会员与模板库后台化：预览门控、轻量首页与 Admin CRUD

- 会员改为永久激活方案；云端文件不再拦截非会员；预览弹窗内导出/打印才校验
- 首页最近文件支持本地记录，登录后与云端合并；移除独立会员页与订阅页
- 模板库入库管理：soon_templates 表、Admin 上传 CRUD、/templates 轻量列表与按需下载

Co-authored-by: Cursor <cursoragent@cursor.com>

backend-web/src/Admin/Controllers/UsersController.php

@@ -7,6 +7,7 @@ use Soon\Api\Core\Db;
 use Soon\Api\Core\Json;
 use Soon\Api\Services\AdminPermission;
 use Soon\Api\Services\AuditService;
+use Soon\Api\Services\MembershipService;
 
 final class UsersController
 {
@@ -270,9 +271,9 @@ final class UsersController
         try {
             $pdo->prepare('UPDATE subscriptions SET status = \'expired\' WHERE user_id = :u AND status = \'active\'')
                 ->execute(['u' => $id]);
-            if ($plan['code'] !== 'free' && $durationDays > 0) {
+            if ($plan['code'] !== 'free') {
                 $now = date('Y-m-d H:i:s');
-                $expires = date('Y-m-d H:i:s', time() + $durationDays * 86400);
+                $expires = MembershipService::subscriptionExpiresAt($durationDays);
                 $pdo->prepare(
                     'INSERT INTO subscriptions (user_id, plan_id, status, started_at, expires_at) '
                     . 'VALUES (:u, :p, \'active\', :sa, :ea)'