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/Livewire/Posts/Index.php

@@ -29,6 +29,8 @@ class Index extends Component
 
     public string $target = '';
 
+    public string $imageUrl = '';
+
     public string $status = 'draft';
 
     public string $scheduledAt = '';
@@ -82,6 +84,7 @@ class Index extends Component
         $this->title = (string) $post->title;
         $this->body = $post->body;
         $this->target = (string) $post->target;
+        $this->imageUrl = (string) $post->image_url;
         $this->status = $post->status;
         $this->scheduledAt = $post->scheduled_at?->format('Y-m-d\TH:i') ?? '';
         $this->showForm = true;
@@ -96,6 +99,7 @@ class Index extends Component
             'title' => ['nullable', 'string', 'max:255'],
             'body' => ['required', 'string'],
             'target' => ['nullable', 'string', 'max:255'],
+            'imageUrl' => ['nullable', 'url', 'max:2048'],
             'status' => ['required', 'string', 'in:' . implode(',', Post::STATUSES)],
             'scheduledAt' => ['nullable', 'date'],
         ]);
@@ -107,6 +111,7 @@ class Index extends Component
             'title' => $validated['title'] ?: null,
             'body' => $validated['body'],
             'target' => $validated['target'] ?: null,
+            'image_url' => $validated['imageUrl'] ?: null,
             'status' => $validated['status'],
             'scheduled_at' => $validated['scheduledAt'] ?: null,
         ]);
@@ -129,6 +134,7 @@ class Index extends Component
         $this->title = '';
         $this->body = '';
         $this->target = '';
+        $this->imageUrl = '';
         $this->status = 'draft';
         $this->scheduledAt = '';
         $this->resetValidation();