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>

themes/sablog/views/show.blade.php

@@ -30,8 +30,8 @@
                     </details>
                 </nav>
             @endif
-        @if($cover = $post->getFirstMedia('cover'))
-            <div class="post-cover"><img src="{{ $cover->getUrl('card') ?: $cover->getUrl() }}" alt="{{ $post->title }}"></div>
+        @if($url = $post->coverImageUrl())
+            <div class="post-cover"><img src="{{ $url }}" alt="{{ $post->title }}"></div>
         @endif
         <h1 class="post-title">{{ $post->title }}</h1>
         <div class="post-meta">