feat: add OpenRouter image provider for higher-quality image generation

Replaces Cloudflare Workers AI (FLUX.1 schnell) as the default image
provider with OpenRouter, which offers FLUX.2 Pro, Seedream 3.0,
Gemini Flash Image, and GPT Image 1 at comparable or better pricing.

- New provider class (3-file split: provider, pricing, support) reuses
  the existing OpenRouter API key and Cloudflare AI Gateway
- Admin settings: new Image Provider dropdown (openrouter/cloudflare),
  model picker shows all available image models from both providers
- Image pipeline auto-selects the configured provider at construction
- Cost logging now uses actual model name instead of hardcoded flux-1-schnell
- Default model changed to black-forest-labs/flux-2-pro ($0.03/image)
- Unit tests for pricing resolution and provider response parsing

Cost impact: FLUX.2 Pro at $0.03/image vs schnell at $0.0007/image —
~43x more expensive per image, but dramatically better quality. At 2
images/article × 1 article/day = ~$1.80/month. Seedream 3.0 ($0.003)
available as a budget alternative.

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

includes/providers/class-openrouter-image-pricing.php

@@ -0,0 +1,79 @@
+<?php
+declare(strict_types=1);
+
+/**
+ * Pricing + model-id resolution helpers for the OpenRouter image provider.
+ *
+ * Mirrors the Cloudflare pricing helper pattern. Each model's cost per image
+ * is a flat rate sourced from OpenRouter's pricing page (April 2026).
+ *
+ * Triggered by: PRAutoBlogger_OpenRouter_Image_Provider (both generate_image()
+ *               and estimate_cost() delegate here).
+ * Dependencies: None — pure calculation + option reads.
+ *
+ * @see class-openrouter-image-provider.php — Caller.
+ * @see class-cloudflare-image-pricing.php  — Sibling pattern for Cloudflare.
+ */
+class PRAutoBlogger_OpenRouter_Image_Pricing {
+
+	/**
+	 * Flat per-image cost (USD) for each supported model, April 2026.
+	 * Update here when OpenRouter reprices or new models launch.
+	 *
+	 * @var array<string, float>
+	 */
+	private const COST_PER_IMAGE = [
+		'black-forest-labs/flux-2-pro'          => 0.03,
+		'black-forest-labs/flux-1.1-pro'        => 0.04,
+		'black-forest-labs/flux-1.1-pro-ultra'  => 0.06,
+		'bytedance/seedream-3.0'                => 0.003,
+		'google/gemini-2.5-flash-preview:image' => 0.01,
+		'openai/gpt-image-1'                    => 0.04,
+	];
+
+	/** @var string Default model when none is configured. */
+	public const DEFAULT_MODEL = 'black-forest-labs/flux-2-pro';
+
+	/**
+	 * Resolve the model identifier from a hint, falling back to site option,
+	 * then to the hardcoded default.
+	 *
+	 * @param string $hint Caller-supplied model identifier (may be empty).
+	 * @return string OpenRouter model id.
+	 */
+	public function resolve_model( string $hint ): string {
+		$hint = trim( $hint );
+		if ( '' === $hint ) {
+			$hint = (string) get_option( 'prautoblogger_image_model', self::DEFAULT_MODEL );
+		}
+		// Validate the model exists in our cost table; fall back to default.
+		if ( ! isset( self::COST_PER_IMAGE[ $hint ] ) ) {
+			return self::DEFAULT_MODEL;
+		}
+		return $hint;
+	}
+
+	/**
+	 * Estimate USD cost for a single image at the given model.
+	 *
+	 * OpenRouter image models use flat per-image pricing, not per-megapixel.
+	 * Width/height are accepted for interface compatibility but don't affect cost.
+	 *
+	 * @param int    $width  Width in pixels (unused for flat pricing).
+	 * @param int    $height Height in pixels (unused for flat pricing).
+	 * @param string $model  Resolved OpenRouter model id.
+	 * @return float USD cost rounded to 6 decimals.
+	 */
+	public function estimate_cost( int $width, int $height, string $model ): float {
+		return round( self::COST_PER_IMAGE[ $model ] ?? self::COST_PER_IMAGE[ self::DEFAULT_MODEL ], 6 );
+	}
+
+	/**
+	 * Get the full model cost table for admin display.
+	 *
+	 * @return array<string, float> Model id => cost per image in USD.
+	 */
+	public static function get_model_costs(): array {
+		return self::COST_PER_IMAGE;
+	}
+}