issue a card with Marqeta and handle JIT funding gateway requests

Verified steps

1. Create a card product in the Marqeta dashboard or via POST /cardproducts, defining the funding source, JIT funding configuration (including your gateway URL), and card program settings.
2. Create a user by POSTing to /users with identifying details; receive a user_token.
3. Issue a card by POSTing to /cards with the user_token and card_product_token; for virtual cards the card is ready immediately, for physical cards it enters a fulfillment queue.
4. Implement your JIT funding webhook endpoint — Marqeta will POST an authorization request to your gateway_url for every transaction; respond within the required timeout (typically under 5 seconds) with a JSON body containing amount and a jit_funding object indicating approval or decline.
5. In your JIT gateway handler, check the user's available balance in your ledger, deduct the amount if sufficient, and return a 200 response with jit_funding.method=pgfs.authorization and the approved amount; return a decline code if insufficient.
6. Reconcile final transaction amounts using Marqeta's clearing webhooks, which may differ from the authorization amount (e.g., for tips or fuel holds).