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/admin/class-settings-fields.php

@@ -63,7 +63,7 @@ class PRAutoBlogger_Settings_Fields {
 			'prautoblogger_images' => [
 				'title'       => __( 'Images', 'prautoblogger' ),
 				'icon'        => 'dashicons-format-image',
-				'description' => __( 'Cloudflare Workers AI credentials and image style controls.', 'prautoblogger' ),
+				'description' => __( 'Image provider, model, and style controls. OpenRouter reuses your existing API key.', 'prautoblogger' ),
 			],
 		];
 	}