Sync sold orders and pull historical price-guide comps from the Reverb API

Steps

1. Authenticate with a bearer/X-Auth-Token as with the listings API
2. GET https://api.reverb.com/api/my/orders/selling on a schedule (cron/poll), filtered by a date range, to pick up new sales
3. Match returned order line items back to internal SKUs via the listing/product identifiers in the payload
4. Query Reverb's separate public Price Guide HAL+JSON endpoints (no auth required) for historical sold-price comps to inform pricing
5. Cache Price Guide responses locally since the endpoint is unauthenticated and rate-sensitive