Triggering a sync

A sync is queued, not immediate. POST /external-syncs/{platform}/sync returns as soon as the job is accepted, so treat 200 as "queued" and poll GET /external-syncs/settings to follow it:

1. status becomes syncing while the job runs.
2. For QuickBooks Online, progress reports percent complete.
3. status returns to idle on success, or failed with an error.message.