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>

tests/Feature/Api/PostsApiTest.php

@@ -142,3 +142,22 @@ test('it includes published_url in the post response', function () {
 
     $response->assertOk()->assertJsonPath('data.published_url', 'https://example.com/my-post');
 });
+
+test('image_url can be set on create and update', function () {
+    $site = Site::factory()->create();
+
+    $response = $this->postJson('/api/posts', [
+        'site_id' => $site->id,
+        'type' => 'blog',
+        'body' => 'content',
+        'image_url' => 'https://example.com/photo.jpg',
+    ]);
+
+    $response->assertCreated()->assertJsonPath('data.image_url', 'https://example.com/photo.jpg');
+
+    $post = Post::latest('id')->first();
+
+    $this->patchJson("/api/posts/{$post->id}", ['image_url' => 'https://example.com/new-photo.jpg'])
+        ->assertOk()
+        ->assertJsonPath('data.image_url', 'https://example.com/new-photo.jpg');
+});