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>

README.md

@@ -26,8 +26,10 @@ Built with Laravel 13, Livewire, and Tailwind CSS.
   (e.g. Reddit's browser automation) don't block the page.
 - Built-in publishers (`app/Support/Publishing/`), one per network, resolved
   by `Post.type`:
-  - **WordPress**: posts directly to a site's WordPress REST API. Also
-    available synchronously from the CLI: `php artisan posts:publish {post_id}`.
+  - **WordPress**: posts directly to a site's WordPress REST API, uploading
+    `Post.image_url` as the featured image first — required to publish a
+    WordPress post, optional for every other post type. Also available
+    synchronously from the CLI: `php artisan posts:publish {post_id}`.
   - **Reddit**: drives a standalone Playwright sidecar at
     `sidecars/reddit-post/` that logs in and creates a post.
   - Anything else: no publisher yet — publish however fits, then report