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>

ARCHITECTURE.md

@@ -138,7 +138,8 @@ prautoblogger/
 │   │   ├── class-image-media-sideloader.php # Imports images into WordPress media library
 │   │   ├── class-cost-tracker.php     # Logs all API costs, enforces budget limits
 │   │   ├── class-logger.php           # Structured logging singleton (error/warning/info/debug)
-│   │   ├── class-pipeline-runner.php  # Orchestrates the full generation pipeline
+│   │   ├── class-article-worker.php   # Single-article generation (content + edit + publish)
+│   │   ├── class-pipeline-runner.php  # Orchestrates pipeline; chains per-article cron jobs
 │   │   ├── class-ga4-client.php       # Google Analytics 4 API client (OAuth + Data API)
 │   │   └── class-metrics-collector.php# Collects post performance data (WP + GA4)
 │   │