feat: env-configurable eBay marketplace, category tree, and currency

EBAY_MARKETPLACE_ID, EBAY_CATEGORY_TREE_ID, and EBAY_CURRENCY can now be
set via environment variables so self-hosters can list on non-US eBay
sites. Defaults are unchanged (EBAY_US / tree 0 / USD), so existing
deployments behave identically. Pairs with
scripts/refresh-category-map.ts for regenerating the offline category
fallback after switching marketplaces.