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/providers/class-runware-image-pricing.php

@@ -15,6 +15,7 @@ declare(strict_types=1);
  * Dependencies: None — pure calculation + option reads.
  *
  * @see class-runware-image-provider.php — Primary caller.
+ * @see class-runware-model-catalog.php — Live model sync; authoritative pricing source.
  * @see class-open-router-image-pricing.php — Sibling pattern for OpenRouter.
  * @see admin/class-image-model-registry.php — Source of truth for model list; keep in sync.
  */