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/show.blade.php

@@ -30,9 +30,9 @@
                     </details>
                 </nav>
             @endif
-            @if($cover = $post->getFirstMedia('cover'))
+            @if($url = $post->coverImageUrl())
                 <div class="post-cover">
-                    <img src="{{ $cover->getUrl('card') ?: $cover->getUrl() }}" alt="{{ $post->title }}">
+                    <img src="{{ $url }}" alt="{{ $post->title }}">
                 </div>
             @endif
             <h1 class="post-title">{{ $post->title }}</h1>