Module helpers/settingsMigration

Migrates legacy flat user-settings keys into the current nested objects:

• caching (boolean) + cacheTtlMinutes + doNotCacheEmptyResults → caching: { enabled, ttlMinutes, doNotCacheEmptyResults }
• trackPriceHistory + priceHistoryMaxPoints → priceTracking: { enabled, maxDataPoints }
• suppliers (array) + disabledSuppliers + excludeNonShippingSuppliers + supplierResultLimit → suppliers: { enabled, disabled, excludeNonShipping, resultLimit }