feat(blog): add SEO fields, scheduling, and representative ownership to blog posts

- Introduced new SEO fields (meta_title, meta_description, primary_keyword, secondary_keywords, faq, internal_links, external_links, reading_time, canonical_url, og_image) to the Blog entity.
- Added scheduling capability with a scheduled_at field to manage automatic publishing of blog posts.
- Implemented representative ownership through a foreign key representation_id in the Blog entity, allowing representatives to manage their own posts.
- Updated BlogController and RepresentationBlogController to handle new fields and ensure proper data handling for SEO and scheduling.
- Created BlogWriter service to encapsulate the logic for applying SEO and scheduling fields to blog entities.
- Added PublishScheduledBlogsMessage and its handler to manage the publishing of scheduled blogs.
- Implemented ScheduledBlogPublisher service to publish drafts whose scheduled_at has arrived, respecting review status.
- Created migration to update the database schema with new fields and constraints.
- Added tests to ensure the correct functionality of new features, including SEO fields, representative scope, and scheduled publishing.

src/Blog/Message/PublishScheduledBlogsMessage.php

@@ -0,0 +1,11 @@
+<?php
+
+namespace App\Blog\Message;
+
+/**
+ * Marker message dispatched every minute by the scheduler. Its handler publishes
+ * any draft blog whose scheduled_at has arrived (and which passed the review gate).
+ */
+final class PublishScheduledBlogsMessage
+{
+}