- Checkout returns 503 CREDITS_DISABLED while OPT_CREDITS_MODE=off, so a
customer can never pay for credits that the pipeline will not consume.
- Packs expose purchases_enabled; the portal hides Buy and says billing is off.
- Structuring an unparsed document (e.g. a scanned PDF with no text layer) now
answers 422 DOCUMENT_NOT_PARSED instead of a generic INVALID_REQUEST.