Refactor force_refresh path to use thread pool execution instead of blocking
the event loop shared with ComfyUI. Key changes:
- Fix 1: Route force_refresh through _initialize_recipe_cache_sync() in thread pool
- Fix 2: Add GIL release points (time.sleep(0)) every 100 files in sync loops
- Fix 3: Move RecipeCache.resort() to thread pool via run_in_executor
- Fix 4: Persist cache automatically after force_refresh
- Fix 5: Increase yield frequency in _enrich_cache_metadata (every recipe)
This eliminates the ~5 minute freeze when rebuilding 30K recipe cache.
Fixes performance issue where ComfyUI became unresponsive during recipe
scanning due to shared Python event loop blocking.