fix: 已解锁后 [paid] 标签裸露——替换为付费内容块

post.rendered 过滤原只在无权限时替换 [paid]；有权限时原始标签直接显示在页面。现在：
- 有权限：替换为 <div class="paid-content"> 付费内容块（带「付费内容」小标签），三套主题补齐样式
- 先去掉 Markdown 转换产生的 <p>[paid]</p> 包裹，避免替换后残留未闭合标签
- 测试：游客与会员均断言不出现裸 [paid]

tests/Feature/MembershipFlowTest.php

@@ -87,21 +87,24 @@ class MembershipFlowTest extends TestCase
             'published_at' => now(),
         ]);
 
-        // 游客：付费部分被替换为 teaser
+        // 游客：付费部分被替换为 teaser，且不出现裸 [paid] 标签
         $this->get('/posts/paid-post.shtml')
             ->assertOk()
             ->assertDontSee('付费隐藏内容')
+            ->assertDontSee('[paid]')
             ->assertSee('本文包含付费内容')
             ->assertSee('登录后查看');
 
-        // 会员：可见
+        // 会员：可见，且 [paid] 标签被替换为付费内容块（不裸露）
         $this->actingAs($this->user)->post('/membership/'.$this->plan->id.'/subscribe');
         $payment = Payment::query()->where('user_id', $this->user->id)->first();
         $this->actingAs($this->user)->post('/pay/sandbox/'.$payment->order_no.'/confirm');
 
         $this->actingAs($this->user)->get('/posts/paid-post.shtml')
             ->assertOk()
-            ->assertSee('付费隐藏内容');
+            ->assertSee('付费隐藏内容')
+            ->assertSee('paid-content')
+            ->assertDontSee('[paid]');
     }
 
     public function test_unlock_post_creates_payment_with_payable_and_grants_access(): void