Uploads every saved item's media and metadata to IPFS, then saves the resulting folder as the drop's draft base URI. Run it after POST /api/v2/drops/{slug}/items/media/save; the drop needs at least one item. It does not change the contract: call POST /api/v2/drops/{slug}/publish once it completes.

Only one upload runs per drop at a time. Calling again while one is running returns that upload's id. Poll GET /api/v2/drops/{slug}/metadata/ipfs/{workflow_execution_id} until status is completed or failed.