Add an image field, required for WordPress posts

Posts get a nullable image_url. WordPressPublisher now downloads it
and uploads it to WordPress's media endpoint before creating the
post, setting it as featured_media — and rejects publishing (without
touching status) if a blog post has no image set, mirroring the
existing title requirement. Every other network treats it as optional.

Not enforced at save time, same as title — you can still draft an
incomplete WordPress post, it just can't be published until an image
is set. Exposed through the UI form (with a thumbnail/""missing""
indicator in the table), the JSON API, and the Python CLI wrapper.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

app/Http/Resources/PostResource.php

@@ -17,6 +17,7 @@ class PostResource extends JsonResource
             'title' => $this->title,
             'body' => $this->body,
             'target' => $this->target,
+            'image_url' => $this->image_url,
             'status' => $this->status,
             'scheduled_at' => $this->scheduled_at,
             'published_url' => $this->published_url,