feat: use xpier media library for uploads and markdown embeds

Store media:ID in post bodies instead of CDN URLs, and keep sablog attach tokens as a fallback.

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

resources/views/list.blade.php

@@ -9,10 +9,10 @@
     <main class="content">
         <h1 class="list-title">{{ $archiveTitle ?? ($category->name ?? '文章列表') }}</h1>
 
-    @if(isset($category) && ($category->description || $category->getFirstMedia('cover')))
+    @if(isset($category) && ($category->description || $category->coverImageUrl()))
         <div class="category-intro">
-            @if($cover = $category->getFirstMedia('cover'))
-                <img src="{{ $cover->getUrl() }}" alt="{{ $category->name }}" class="category-cover">
+            @if($url = $category->coverImageUrl())
+                <img src="{{ $url }}" alt="{{ $category->name }}" class="category-cover">
             @endif
             <div>
                 @if($category->description)