Make the image optional for WordPress posts too

Some blog posts just don't have one. WordPressPublisher no longer
rejects publishing a type=blog post without image_url — it uploads
and attaches it as the featured image when present, and simply skips
that step (no featured_media set) otherwise.

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

README.md

@@ -26,10 +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, 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}`.
+  - **WordPress**: posts directly to a site's WordPress REST API. If
+    `Post.image_url` is set (optional, for every post type), it's uploaded
+    and used as the featured image. 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