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

themes/sablog/assets/style.css

@@ -144,3 +144,14 @@ a:hover { color: #f60; text-decoration: underline; }
 .toc-level-4, .toc-level-5, .toc-level-6 { padding-left: 28px; }
 
 .sr-only { position: absolute; width: 1px; height: 1px; padding: 0; margin: -1px; overflow: hidden; clip: rect(0 0 0 0); white-space: nowrap; border: 0; }
+
+/* Paywall */
+.paywall { border: 1px solid #e0e0e0; border-radius: 8px; padding: 24px 20px; background: #fafafa; text-align: center; }
+.paywall-cover img { max-width: 100%; border-radius: 6px; margin-bottom: 14px; }
+.paywall-title { font-size: 20px; margin: 0 0 10px; }
+.paywall-excerpt { color: #666; font-size: 14px; margin: 0 0 14px; }
+.paywall-price { font-size: 22px; font-weight: 800; color: #f60; margin: 0 0 8px; }
+.paywall-hint { color: #888; font-size: 13px; margin: 0 0 16px; }
+.paywall-actions .btn { margin: 0 6px; }
+.paid-teaser { border: 1px dashed #f5c6cb; background: #fdecea; color: #c62828; border-radius: 6px; padding: 12px 14px; margin: 12px 0; font-size: 14px; }
+.paid-teaser .btn { margin-left: 8px; }