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/Requests/StorePostRequest.php

@@ -21,6 +21,7 @@ class StorePostRequest extends FormRequest
             'title' => ['nullable', 'string', 'max:255'],
             'body' => ['required', 'string'],
             'target' => ['nullable', 'string', 'max:255'],
+            'image_url' => ['nullable', 'url', 'max:2048'],
             'status' => ['sometimes', 'string', 'in:' . implode(',', Post::STATUSES)],
             'scheduled_at' => ['nullable', 'date'],
         ];

app/Http/Requests/UpdatePostRequest.php

@@ -21,6 +21,7 @@ class UpdatePostRequest extends FormRequest
             'title' => ['nullable', 'string', 'max:255'],
             'body' => ['sometimes', 'required', 'string'],
             'target' => ['nullable', 'string', 'max:255'],
+            'image_url' => ['nullable', 'url', 'max:2048'],
             'status' => ['sometimes', 'string', 'in:' . implode(',', Post::STATUSES)],
             'scheduled_at' => ['nullable', 'date'],
             'published_url' => ['nullable', 'string', 'max:255'],