Submits an order for fulfillment or reschedules its submit date.

Submitting an order makes it eligible for the operations team to begin fulfillment. Once submitted, the order enters the fulfillment pipeline and edit restrictions apply:

• During picking, order items cannot be changed.
• Once packed, no fields on the order can be changed.

This endpoint serves two purposes:

• Submit immediately — pass a submit_at timestamp in the past (or now) to release the order into the fulfillment queue right away.
• Reschedule the submit date — even though submit_at can be set when creating an order, this is the simplest way to change it after the fact.

When submit_at is in the future, inventory is reserved immediately and the order is released into the fulfillment queue at the scheduled time.