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>

tests/unit/Providers/OpenRouterImagePricingTest.php

@@ -0,0 +1,82 @@
+<?php
+/**
+ * Tests for PRAutoBlogger_OpenRouter_Image_Pricing.
+ *
+ * Validates model resolution fallback logic and per-image cost estimation.
+ *
+ * @package PRAutoBlogger\Tests\Providers
+ */
+
+namespace PRAutoBlogger\Tests\Providers;
+
+use PRAutoBlogger\Tests\BaseTestCase;
+use Brain\Monkey\Functions;
+
+class OpenRouterImagePricingTest extends BaseTestCase {
+
+	protected function setUp(): void {
+		parent::setUp();
+		Functions\when( 'get_option' )->alias( function ( $key, $default = false ) {
+			static $options = [];
+			return $options[ $key ] ?? $default;
+		} );
+	}
+
+	/** Default model constant is FLUX.2 Pro. */
+	public function test_default_model_constant(): void {
+		$this->assertSame(
+			'black-forest-labs/flux-2-pro',
+			\PRAutoBlogger_OpenRouter_Image_Pricing::DEFAULT_MODEL
+		);
+	}
+
+	/** Empty hint resolves to default model. */
+	public function test_resolve_model_empty_hint_uses_default(): void {
+		$pricing = new \PRAutoBlogger_OpenRouter_Image_Pricing();
+		$model   = $pricing->resolve_model( '' );
+		$this->assertSame( 'black-forest-labs/flux-2-pro', $model );
+	}
+
+	/** Valid hint resolves to itself. */
+	public function test_resolve_model_valid_hint(): void {
+		$pricing = new \PRAutoBlogger_OpenRouter_Image_Pricing();
+		$model   = $pricing->resolve_model( 'bytedance/seedream-3.0' );
+		$this->assertSame( 'bytedance/seedream-3.0', $model );
+	}
+
+	/** Unknown model falls back to default. */
+	public function test_resolve_model_unknown_falls_back_to_default(): void {
+		$pricing = new \PRAutoBlogger_OpenRouter_Image_Pricing();
+		$model   = $pricing->resolve_model( 'nonexistent/model-999' );
+		$this->assertSame( 'black-forest-labs/flux-2-pro', $model );
+	}
+
+	/** FLUX.2 Pro costs $0.03/image. */
+	public function test_estimate_cost_flux2_pro(): void {
+		$pricing = new \PRAutoBlogger_OpenRouter_Image_Pricing();
+		$cost    = $pricing->estimate_cost( 1200, 632, 'black-forest-labs/flux-2-pro' );
+		$this->assertEqualsWithDelta( 0.03, $cost, 0.000001 );
+	}
+
+	/** Seedream costs $0.003/image. */
+	public function test_estimate_cost_seedream(): void {
+		$pricing = new \PRAutoBlogger_OpenRouter_Image_Pricing();
+		$cost    = $pricing->estimate_cost( 1200, 632, 'bytedance/seedream-3.0' );
+		$this->assertEqualsWithDelta( 0.003, $cost, 0.000001 );
+	}
+
+	/** Unknown model in estimate_cost falls back to default pricing. */
+	public function test_estimate_cost_unknown_model(): void {
+		$pricing = new \PRAutoBlogger_OpenRouter_Image_Pricing();
+		$cost    = $pricing->estimate_cost( 1200, 632, 'unknown/model' );
+		$this->assertEqualsWithDelta( 0.03, $cost, 0.000001 );
+	}
+
+	/** get_model_costs returns a non-empty map. */
+	public function test_get_model_costs(): void {
+		$costs = \PRAutoBlogger_OpenRouter_Image_Pricing::get_model_costs();
+		$this->assertIsArray( $costs );
+		$this->assertArrayHasKey( 'black-forest-labs/flux-2-pro', $costs );
+		$this->assertGreaterThan( 0, count( $costs ) );
+	}
+}