Transaction status lookup

Returns status and details for a transaction by `txid` (transaction id or first UDF). Requires merchant bearer token (`Authorization: Bearer …`). Do **not** send a `Request-for` header — that path is for internal services only and is not part of this merchant API. **Response paths (merchant):** 1. **Authentication** — 401/403 if bearer token, site, or client IP fails validation. 2. **Site config** — 401 if lookup is disabled (`1402`, `{txn, errors}`); 500 if payment config is missing (`1403`). 3. **Database** — 500 if the transaction query fails (`1401`). 4. **Not found** — HTTP **404** with `txn` and `errors` (code **1402**, `transaction not found!`). No payment fields are returned. 5. **Success** — HTTP **200** with gateway transaction fields and `errors: []` (see response example). Additional keys may appear depending on payment method, status, and site config (e.g. `refundable_amount`, `sender_name`, `sender_bank`, `sender_email`, Interac references, `sender_names`, `card_id`, `gateway_response`, `child_txn`).

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