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/class-prautoblogger.php

@@ -95,6 +95,13 @@ class PRAutoBlogger {
 		// Hostinger's 120-second web-server connection timeout.
 		add_action( 'prautoblogger_manual_generation', [ $this->executor, 'on_manual_generation' ] );
 
+		// Chained article generation — each queued article fires as its own
+		// cron event so it gets a fresh PHP process and execution time budget.
+		add_action(
+			PRAutoBlogger_Pipeline_Runner::CRON_ACTION,
+			[ $this->executor, 'on_process_article_queue' ]
+		);
+
 		$registry = $this->executor->get_model_registry();
 		add_action( 'prautoblogger_refresh_model_registry', [ $registry, 'refresh' ] );
 	}