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-prautoblogger.php

@@ -232,9 +232,12 @@ class PRAutoBlogger {
 			update_option( 'prautoblogger_migrated_style_suffix_v4', '1' );
 		}
 
+		// v0.9.0: flip legacy default image model to Runware schnell.
+		PRAutoBlogger_Activator::migrate_default_image_v090();
+
 		// One-time migration: re-wrap existing encrypted values with "enc:" prefix.
 		if ( ! get_option( 'prautoblogger_migrated_enc_prefix' ) ) {
-			$enc_options = [ 'prautoblogger_openrouter_api_key', 'prautoblogger_ga4_credentials_json' ];
+			$enc_options = [ 'prautoblogger_openrouter_api_key', 'prautoblogger_ga4_credentials_json', 'prautoblogger_runware_api_key' ];
 			foreach ( $enc_options as $opt ) {
 				$val = get_option( $opt, '' );
 				if ( '' !== $val && ! PRAutoBlogger_Encryption::is_encrypted( $val ) ) {