Complete buyer delivery setup

Completes the buyer delivery setup process for a connection. This endpoint performs several operations:

1. Validates the connection_id or resolves it from invitation_id
2. Finds the seller account and their flows that deliver to this buyer
3. Updates or creates a buyer flow with the seller as a source
4. Updates seller flows with the buyer’s flow_id mapping
5. Sends notification emails to seller account users
6. Marks the connection as having buyer delivery setup complete

Note: Either connection_id or invitation_id must be provided.

Version Note:

• In the first version of the onboarding modal, this endpoint is called directly by the frontend.
• In the second version, this endpoint will be called automatically by the backend when the onboard status changes to completed_flows_sync.