feat: chain per-article cron jobs to eliminate execution time ceiling

The pipeline previously ran all articles sequentially in a single PHP
process. On Hostinger, this timed out at ~120s — enough for 1 article
but not 2+. Now the pipeline generates article 1 in-process, then
queues articles 2..N as individual WP-Cron events. Each article gets
its own PHP process with its own execution time budget, so we can
generate any number of articles regardless of server time limits.

Architecture:
- Pipeline_Runner: orchestrate (collect+analyze+score) + first article,
  then serialize remaining ideas to wp_options and chain cron events
- Article_Worker (new): focused single-article worker extracted from
  the pipeline loop — content gen + editorial + publish
- Executor: new on_process_article_queue() handler for chained events;
  status poller re-schedules orphaned queue jobs automatically
- Article_Idea: added to_array() for queue serialization

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

includes/models/class-article-idea.php

@@ -84,4 +84,25 @@ class PRAutoBlogger_Article_Idea {
 	public function get_target_keywords(): array {
 		return $this->target_keywords;
 	}
+
+	/**
+	 * Serialize to array for storage in options/transients.
+	 *
+	 * Used by the pipeline queue to persist ideas across chained cron jobs.
+	 *
+	 * @return array<string, mixed> Constructor-compatible array.
+	 */
+	public function to_array(): array {
+		return [
+			'topic'           => $this->topic,
+			'article_type'    => $this->article_type,
+			'suggested_title' => $this->suggested_title,
+			'summary'         => $this->summary,
+			'score'           => $this->score,
+			'analysis_id'     => $this->analysis_id,
+			'source_ids'      => $this->source_ids,
+			'key_points'      => $this->key_points,
+			'target_keywords' => $this->target_keywords,
+		];
+	}
 }