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>

database/migrations/2026_07_21_195849_add_image_url_to_posts_table.php

@@ -0,0 +1,28 @@
+<?php
+
+use Illuminate\Database\Migrations\Migration;
+use Illuminate\Database\Schema\Blueprint;
+use Illuminate\Support\Facades\Schema;
+
+return new class extends Migration
+{
+    /**
+     * Run the migrations.
+     */
+    public function up(): void
+    {
+        Schema::table('posts', function (Blueprint $table) {
+            $table->string('image_url')->nullable()->after('target');
+        });
+    }
+
+    /**
+     * Reverse the migrations.
+     */
+    public function down(): void
+    {
+        Schema::table('posts', function (Blueprint $table) {
+            $table->dropColumn('image_url');
+        });
+    }
+};