Receives a Reserve Funding Request from the external reserve funding service. Validates the payload (merchant MID and department are resolved against HQ; the department must be a Reserve department, indicator 2), enforces idempotency on externalRequestId (duplicate returns HTTP 200 with duplicate=true), downloads and stores the signed document internally, and saves the request with Pending status for review in the Reserve Funding Requests module. Returns HTTP 201 with the internal requestId; validation errors are returned together as HTTP 400 (errors). Requires the manage-reserve-funding-requests module and the "allow Reserve Funding Management API" role option (otherwise HTTP 403).