feat: 前台 paywall——整篇锁定渲染付费墙，付费块替换为 teaser

- membership/post.rendered 过滤：整篇锁定（members_only）渲染 paywall 视图（封面+标题+价格+单篇解锁/开通会员按钮，游客引导登录）；[paid] 块替换为带解锁按钮的 teaser；替换改用 preg_replace_callback 避免 $ 转义问题
- 视图放 resources/views/membership/（主题同名文件可覆盖），两主题补充 .paywall/.paid-teaser 样式
- 修复泄漏：excerpt_or_fallback 对 members_only 文章不再回退截取正文，防止内容泄漏到 paywall 与 SEO meta
- 测试：更新 teaser 断言，新增 guest 见 paywall、登录用户见解锁/会员按钮

plugins/neatstudio.membership/src/ServiceProvider.php

@@ -53,21 +53,21 @@ class ServiceProvider extends PluginServiceProvider
             }
         }, 10);
 
-        // 付费文章：[paid]...[/paid] 对无权限访客隐藏
+        // 付费文章：[paid]...[/paid] 对无权限访客隐藏为 teaser；整篇锁定渲染 paywall
         $manager->addFilter('post.rendered', function (string $html, \App\Models\Post $post) {
             $service = app(Services\MembershipService::class);
             $user = auth()->user();
 
             if (preg_match('/\[paid\](.*?)\[\/paid\]/s', $html) && ! $service->canReadPaidContent($user, $post)) {
-                $html = preg_replace(
+                $html = preg_replace_callback(
                     '/\[paid\](.*?)\[\/paid\]/s',
-                    '<div class="paid-teaser">（付费内容已隐藏，开通会员或单篇解锁后可见）</div>',
+                    fn () => view('membership.paid-teaser', ['post' => $post])->render(),
                     $html
                 );
             }
 
             if (! $service->canReadPost($user, $post)) {
-                return '<div class="paid-teaser">（本文章为会员专享，开通会员后可见）</div>';
+                return view('membership.paywall', ['post' => $post])->render();
             }
 
             return $html;