v0.9.0: Runware FLUX.1 schnell as default image provider

New Runware image backend at ~$0.0006/image (FLUX.1 schnell) — 65× cheaper
than Gemini 2.5 Flash Image. Provider mirrors OpenRouter's split: interface
+ provider + support + pricing + batch, all under the 300-line cap. Batch
class uses curl_multi for true parallel A/B dispatch — wall-clock drops to
the slowest single image instead of the sum.

Changes:
- Add PRAutoBlogger_Runware_Image_Provider + Pricing + Support + Batch
- Flip PRAUTOBLOGGER_DEFAULT_IMAGE_PROVIDER/MODEL to runware:100@1
- Add prautoblogger_runware_api_key settings field (encrypted at rest)
- v0.9.0 one-time migration flips legacy default model + re-derives provider
- Wire runware branch into Image_Pipeline::create_default_provider()
- Add runware:100@1 and runware:101@1 to Image_Model_Registry (defaults top)
- Update image prompt builder: no more anthropomorphized molecules;
  prefer concrete human-scale scenes (gym bros, doctors, scientists)
- ARCHITECTURE.md + CHANGELOG.md updated with new integration + decision
- Unit tests for pricing, support, provider; update model registry test

Cost impact at 100 images/day (50 articles × A/B):
- Before (Gemini via OpenRouter): $3.90/day ≈ $117/mo
- After  (FLUX schnell via Runware): $0.06/day ≈ $1.80/mo
Net saving: ~$115/mo

includes/class-activator.php

@@ -242,4 +242,29 @@ class PRAutoBlogger_Activator {
 
 		update_option( 'prautoblogger_migrated_run_id_meta_v081', '1' );
 	}
+
+	/**
+	 * v0.9.0 migration: flip legacy default image model to Runware schnell
+	 * and re-derive provider. CEO decision 2026-04-21 — 65× cheaper than
+	 * Gemini Nano Banana. Explicit user selections are preserved.
+	 *
+	 * Side effects: may update `prautoblogger_image_model`,
+	 *               `prautoblogger_image_provider`, and sets the
+	 *               `prautoblogger_migrated_default_image_v090` flag.
+	 */
+	public static function migrate_default_image_v090(): void {
+		if ( get_option( 'prautoblogger_migrated_default_image_v090' ) ) {
+			return;
+		}
+		$curr = (string) get_option( 'prautoblogger_image_model', '' );
+		if ( in_array( $curr, [ '', 'google/gemini-2.5-flash-image' ], true ) ) {
+			update_option( 'prautoblogger_image_model', 'runware:100@1' );
+		}
+		$final = (string) get_option( 'prautoblogger_image_model', 'runware:100@1' );
+		$prov  = PRAutoBlogger_Image_Model_Registry::provider_for( $final );
+		if ( '' !== $prov ) {
+			update_option( 'prautoblogger_image_provider', $prov );
+		}
+		update_option( 'prautoblogger_migrated_default_image_v090', '1' );
+	}
 }