Applies the completion already present in the match's scoring event log (a durably flushed match_completed marker) synchronously, so a client that submits results and immediately refetches sees the match completed instead of racing the async projection job. The completion is read from the log, never from the caller — a request cannot invent one.

Score rows, broadcast and partner webhooks stay with the async projection job; the sequence watermarks make both paths converge idempotently in either order.

Idempotent: an already-completed match responds 204 without change, so a retry after a timed-out-but-successful call is safe.