Unified status poll for bulk export jobs. Handles both contact
exports (exp_-prefixed IDs, from
POST /v1/buyer-intent/contacts/export) and company exports
(cexp_-prefixed IDs, from POST /v1/buyer-intent/companies/export)
— the ID prefix selects the handler.

Poll until status is ready or failed:

• processing — job still running. A job stuck for more than 10
  minutes is automatically marked failed (and refunded under the
  new pricing).
• ready — includes a 24-hour signed download_url to the JSONL
  result file (auto-deleted after 24 hours).
• failed — includes a generic error message and any refunded
  credits.

Credit cost: free (status reads are not charged).