Create a channel order (synchronous)

Validates and synchronously materializes a channel order: checks the channel is active; optionally links an existing customer by customer_id (the order is left unidentified when omitted); validates every item SKU against an active product and every payment method against an active payment method; cross-checks the subtotal against the sum of item totals; then persists the order, its items, addresses, mirrored payments and bootstrap timeline event in a single transaction. A duplicate (same external_order_id + channel) is rejected with 409. Authenticated as a channel service account.