feat: parallel image generation via curl_multi

Both Image A and Image B now fire concurrently through a single
generate_image_batch() call instead of sequentially. Total wall-clock
time equals the slowest image (~20s), not the sum (~40s), which
eliminates the Image B timeout problem on Hostinger where
max_execution_time killed the cron before the second request finished.

New files:
- class-open-router-image-batch.php — curl_multi orchestrator
- generate_image_batch() added to image provider interface

Pipeline refactored: prompts built upfront, single batch call, results
processed per-key. Cloudflare provider gets a sequential fallback.
Tests updated for batch API. All files under 300-line cap.

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

includes/providers/class-cloudflare-image-provider.php

@@ -174,6 +174,28 @@ class PRAutoBlogger_Cloudflare_Image_Provider implements PRAutoBlogger_Image_Pro
 		);
 	}
 
+	/**
+	 * Sequential fallback — Cloudflare Workers AI does not support batching.
+	 *
+	 * {@inheritDoc}
+	 */
+	public function generate_image_batch( array $requests ): array {
+		$results = [];
+		foreach ( $requests as $key => $req ) {
+			try {
+				$results[ $key ] = $this->generate_image(
+					$req['prompt'],
+					$req['width'],
+					$req['height'],
+					$req['options'] ?? []
+				);
+			} catch ( \Throwable $e ) {
+				$results[ $key ] = [ 'error' => $e->getMessage() ];
+			}
+		}
+		return $results;
+	}
+
 	/**
 	 * Estimate the USD cost of a single generation call.
 	 *