Fix false stall detection for multi-article chained generation

The frontend status poller measured elapsed time from `started` (set once
at the beginning of the run) and declared a stall at 300s.  Three articles
take ~6 minutes total, so the poller always false-alarmed on article 3.

Now every stage broadcast writes `last_updated` into the transient, and
the stall check compares idle time since the last update — not since the
run started.  This lets N-article runs complete without false timeout as
long as each individual article finishes within 5 minutes.

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

includes/core/class-article-worker.php

@@ -137,7 +137,8 @@ class PRAutoBlogger_Article_Worker {
 		$transient_key = 'prautoblogger_generation_status';
 		$current       = get_transient( $transient_key );
 		if ( is_array( $current ) && 'running' === ( $current['status'] ?? '' ) ) {
-			$current['stage'] = $stage;
+			$current['stage']        = $stage;
+			$current['last_updated'] = time();
 			set_transient( $transient_key, $current, 600 );
 		}
 	}