Mark a sprint item done. Optional body: {task_id}.

a2a027cf — timeout-safe / idempotent / observable completion over HTTP. correlation_id may be supplied in the body (or is minted here) and is always echoed back on both success and the 409 conflict body below, so a client that timed out waiting on this response can correlate a retry with the original attempt's server-side logs. The underlying db_module.complete_sprint_item call is itself idempotent for the "already done" case (see its docstring) -- retrying this endpoint after a client-side timeout returns 200 with completion_outcome= "already_committed" rather than a misleading 409, UNLESS a genuinely different, conflicting terminal status won a real race, which still reports 409 exactly as before.