Payout Cycle Clear

#1365: lift the HOLD of a scheduled monthly cycle (global admin). Body: {month "YYYY-MM", reason, mode? ("on", default | "dry")}. A scheduled real cycle that started but did not finish (an idempotency conflict, an exception, a crash mid-run) holds its month: the scheduler runs nothing more that month — no hourly re-run that would pay sellers who newly reach the minimum — until this clears it. After the clear the NEXT tick runs that month's cycle again (a full run). Clear only once the cause is fixed; the alternative is to leave the hold and run POST /billing/payouts/run by hand (which never counts as the month's cycle). Emits marketplace.seller.payout_cycle_cleared {month, mode, reason, by}. 404 no_cycle_hold when the month has none; 409 cycle_already_done when the month's cycle finished; 409 payout_run_in_progress while a payout run holds the run lock — the clear itself runs under that lock, so it can never drop the started hold of a run that is still in flight.