fix: retry transient eBay errors (25001 / 5xx) during publish

eBay's Inventory API intermittently fails with errorId 25001 ('A system
error has occurred. Core Inventory Service internal error') or a bare
5xx. These are eBay-side blips that normally succeed on retry, but the
publish pipeline surfaced them immediately as 'Inventory item failed
(500)'.

- All five eBay write calls (inventory PUT x2, offer create, offer
  update, publish) now retry up to 2 times with 1.5s/3s backoff when
  the response is a 5xx or contains errorId 25001
- Retries are logged loudly so persistent eBay outages stay visible
- 25001 gets a user-facing hint (wait and repost) if retries exhaust

Fixes #16