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>

resources/views/livewire/posts/index.blade.php

@@ -11,6 +11,7 @@
     <table class="w-full bg-white border border-gray-200 rounded">
         <thead>
             <tr class="text-left border-b border-gray-200">
+                <th class="p-3">Image</th>
                 <th class="p-3">Site</th>
                 <th class="p-3">Type</th>
                 <th class="p-3">Title</th>
@@ -23,6 +24,15 @@
         <tbody>
             @foreach ($posts as $post)
                 <tr wire:key="post-{{ $post->id }}" class="border-b border-gray-100">
+                    <td class="p-3">
+                        @if ($post->image_url)
+                            <img src="{{ $post->image_url }}" alt="" class="w-10 h-10 object-cover rounded">
+                        @elseif ($post->type === 'blog')
+                            <span class="text-xs text-amber-600">missing</span>
+                        @else
+                            <span class="text-gray-400">—</span>
+                        @endif
+                    </td>
                     <td class="p-3">{{ $post->site->name }}</td>
                     <td class="p-3">{{ ucfirst($post->type) }}</td>
                     <td class="p-3">{{ $post->title ?: \Illuminate\Support\Str::limit($post->body, 40) }}</td>
@@ -106,6 +116,14 @@
                     @error('target') <span class="text-red-600 text-sm">{{ $message }}</span> @enderror
                 </div>
 
+                <div>
+                    <label class="block text-sm font-medium">
+                        Image URL {{ $type === 'blog' ? '(required to publish to WordPress)' : '(optional)' }}
+                    </label>
+                    <input type="url" wire:model="imageUrl" placeholder="https://..." class="w-full border rounded p-2">
+                    @error('imageUrl') <span class="text-red-600 text-sm">{{ $message }}</span> @enderror
+                </div>
+
                 <div>
                     <label class="block text-sm font-medium">Status</label>
                     <select wire:model="status" class="w-full border rounded p-2">