feat: Live Runware model catalog sync — v0.15.0

Implements daily WP-Cron sync from Runware's /v1/models endpoint with 24h TTL caching,
on-demand AJAX refresh button in PRAdmin (Images settings), smart fallback logic, and
pricing merge from the authoritative cost table.

## Key additions:
- class-runware-model-catalog.php: ~300 lines, fetches/normalizes/caches models,
  exposes sync()/get_models()/get_last_synced_at()/is_stale() public API
- AJAX handler: prautoblogger_sync_runware_models_now with nonce + manage_options check
- Daily cron registered on activation, unscheduled on deactivation
- Caching strategy: <24h fresh → use cache; stale → sync; sync fails → fallback
- Pricing merge: costs from class-runware-image-pricing.php injected on every sync
- Fallback never returns empty array (hardcoded 15-model list)
- Unit tests: sync success/failure, stale cache, fallback behavior, pricing merge

## File changes:
- class-image-model-registry.php: refactored to delegate Runware to catalog,
  extracted get_runware_fallback_models() and get_openrouter_models()
- class-settings-fields-extended.php: added runware_catalog_sync field type
  for status display + Sync button in Images settings section
- class-prautoblogger.php: registered daily cron + AJAX hook
- class-activator.php: schedule prautoblogger_sync_runware_models on activation
- class-deactivator.php: unschedule on deactivation
- class-ajax-handlers.php: added on_ajax_sync_runware_models_now() handler
- class-runware-image-pricing.php: added @see cross-reference to catalog
- ARCHITECTURE.md: updated Runware row in External API Integrations table
- prautoblogger.php: version bump to 0.15.0
- CHANGELOG.md: detailed entry describing the fix and why it was necessary

Fixes the pattern from v0.13.8 and v0.14.0 where hardcoded Runware model
expansion without pricing class updates led to silent fallback-to-schnell bugs.

includes/admin/class-settings-fields-extended.php

@@ -259,5 +259,13 @@ class PRAutoBlogger_Settings_Fields_Extended {
 				'description' => __( 'When the provider rejects an image prompt as NSFW, retry once with a generic fallback scene built from the article title. Disable to fail fast if the filter gets trigger-happy.', 'prautoblogger' ),
 			),
 		);
+			array(
+				'id'          => 'prautoblogger_runware_model_catalog',
+				'label'       => __( 'Runware Model Catalog', 'prautoblogger' ),
+				'type'        => 'runware_catalog_sync',
+				'section'     => 'prautoblogger_images',
+				'description' => __( 'Live model catalog sync status and on-demand refresh.', 'prautoblogger' ),
+			),
+		);
 	}
 }