V21 Round-Robin Service Pool

- Provider Service IDs are selected cyclically: 1,2,3,...N,1,2,...
- The next request starts after the provider Service ID used by the previous order for that service.
- IDs with 5 or more non-final orders (pending/processing/in-progress etc.) are skipped.
- If every configured ID has 5+ active/pending orders, the user is told that order traffic is high and to try again later.
- Completed/cancelled/failed/partial orders do not count toward the 5-order limit.
- Both bot/webhook ordering and api/order.php use the same round-robin rule.