HSN service

Color Papers HSN engine (ported from CPDesk; source of truth). Running · 12150 HSN codes.

`POST /v1/hsn/external/batch`	Partner API for crowcrowcrow.com (header `x-api-key`), same contract as CPDesk / Master
`GET /v1/hsn/external/health`	Public health check (open)
`/v1/hsn-finder`, `/v1/hsn-screen`, `/v1/hsn-suggestions`, `/v1/hsn-nightly`	CPDesk panel HSN screens (CPDesk login)

People: use the HSN page in the repricer dashboard — repricer.crowcrowcrow.tech/hsn.