Implement Lithic ASA (Authorization Stream Access) decisioning endpoint

Steps

1. Register a publicly reachable HTTPS endpoint using POST /v1/responder_endpoints with the type set to AUTH_STREAM_ACCESS and the url of your handler
2. Receive inbound POST requests from Lithic during each card authorization; parse the JSON body for amounts, merchant, card, and authentication fields
3. Evaluate decisioning logic and return HTTP 200 with a JSON body containing the result field set to one of the valid enum values
4. Use APPROVED to approve, CHALLENGE to trigger cardholder authentication; any other value declines — use the most specific decline reason (e.g. UNAUTHORIZED_MERCHANT, INSUFFICIENT_FUNDS, VELOCITY_EXCEEDED, SUSPECTED_FRAUD, AVS_INVALID, CARD_PAUSED)
5. Respond within 3 seconds — Lithic times out and declines the transaction after 6 seconds with no response
6. Optionally return avs_result (MATCH, MATCH_ZIP_ONLY, MATCH_ADDRESS_ONLY, FAIL) and approved_amount for partial approvals alongside result