Recomputes the field definitions after a client-side change to one or more fields on an expense report. Send the current fields payload as the request body — typically after a field marked requiresUpdate: true was edited — and the response returns the same shape with derived and dependent fields refreshed. If the body's id field carries an existing expense-report id the server uses it to load report context; otherwise the body alone is the source of truth. Unlike the GET variant, no fields are seeded from server-side records.