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/membership/paywall.blade.php

@@ -7,9 +7,9 @@
 @endphp
 
 <div class="paywall" role="note" aria-label="付费内容">
-    @if($cover = $post->getFirstMedia('cover'))
+    @if($url = $post->coverImageUrl())
         <div class="paywall-cover">
-            <img src="{{ $cover->getUrl('card') ?: $cover->getUrl() }}" alt="{{ $post->title }}">
+            <img src="{{ $url }}" alt="{{ $post->title }}">
         </div>
     @endif