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/interface-image-provider.php

@@ -83,6 +83,37 @@ interface PRAutoBlogger_Image_Provider_Interface {
 	 */
 	public function get_provider_name(): string;
 
+	/**
+	 * Generate multiple images in parallel (or sequentially as fallback).
+	 *
+	 * Each entry in $requests describes one image to generate. The return array
+	 * is indexed identically — callers can zip input and output by key.
+	 *
+	 * Implementations that support concurrent HTTP (e.g. curl_multi) SHOULD
+	 * override the default sequential behaviour for a wall-clock speedup.
+	 *
+	 * Side effects: HTTP requests (possibly concurrent), Logger lines per image.
+	 *
+	 * @param array<string, array{
+	 *     prompt: string,
+	 *     width: int,
+	 *     height: int,
+	 *     options?: array{seed?: int, steps?: int, model?: string},
+	 * }> $requests Keyed image generation requests.
+	 *
+	 * @return array<string, array{
+	 *     bytes: string,
+	 *     mime_type: string,
+	 *     width: int,
+	 *     height: int,
+	 *     model: string,
+	 *     seed: ?int,
+	 *     cost_usd: float,
+	 *     latency_ms: int,
+	 * }|array{error: string}> Each key maps to either image data or an error.
+	 */
+	public function generate_image_batch( array $requests ): array;
+
 	/**
 	 * Verify the provider credentials are present and the API is reachable.
 	 *