Manually trigger a settlement run

Runs the end-of-day settlement for a specified date on demand. This is idempotent – if the date has already been successfully settled, the existing record is returned without reprocessing.

Use cases:

• Re-running a failed batch (e.g. after a database outage).
• Settling a back-date after delayed transaction processing.
• Smoke-testing the settlement pipeline in staging.

The scheduled job automatically covers yesterday at 23:59 UTC; this endpoint is for operational override only.