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

tests/Feature/MembershipFlowTest.php

@@ -87,11 +87,12 @@ class MembershipFlowTest extends TestCase
             'published_at' => now(),
         ]);
 
-        // 游客：付费部分被替换
+        // 游客：付费部分被替换为 teaser
         $this->get('/posts/paid-post.shtml')
             ->assertOk()
             ->assertDontSee('付费隐藏内容')
-            ->assertSee('付费内容已隐藏');
+            ->assertSee('本文包含付费内容')
+            ->assertSee('登录后查看');
 
         // 会员：可见
         $this->actingAs($this->user)->post('/membership/'.$this->plan->id.'/subscribe');
@@ -184,4 +185,43 @@ class MembershipFlowTest extends TestCase
 
         $this->assertSame(0, Payment::query()->count());
     }
+
+    public function test_members_only_post_shows_paywall_to_guest(): void
+    {
+        $post = Post::create([
+            'title' => '会员专享文章',
+            'slug' => 'members-only-post',
+            'content' => '这是会员才能看到的正文内容',
+            'content_format' => 'markdown',
+            'status' => 'published',
+            'published_at' => now(),
+            'meta' => ['members_only' => true, 'price' => 500],
+        ]);
+
+        $this->get('/posts/members-only-post.shtml')
+            ->assertOk()
+            ->assertSee('登录后解锁')
+            ->assertSee('单篇解锁')
+            ->assertSee('会员专享文章')
+            ->assertDontSee('这是会员才能看到的正文内容');
+    }
+
+    public function test_members_only_post_paywall_shows_unlock_buttons_to_user(): void
+    {
+        $post = Post::create([
+            'title' => '会员专享文章2',
+            'slug' => 'members-only-post-2',
+            'content' => '正文内容不可见',
+            'content_format' => 'markdown',
+            'status' => 'published',
+            'published_at' => now(),
+            'meta' => ['members_only' => true, 'price' => 9900],
+        ]);
+
+        $this->actingAs($this->user)->get('/posts/members-only-post-2.shtml')
+            ->assertOk()
+            ->assertSee('单篇解锁')
+            ->assertSee('开通会员')
+            ->assertDontSee('正文内容不可见');
+    }
 }