Import from platform (sync-when-fast, async-when-slow)

Imports clients from the third-party platform. This endpoint uses a hybrid approach: it queues the import job and waits up to 10 seconds for completion. For small imports (5-20 clients), the job typically completes within this window and returns 201 Created with the final result. For larger imports that take longer, it returns 202 Accepted with a Location header pointing to the import job status endpoint. If importAll query parameter is true, imports all available clients and ignores the request body. Otherwise, imports only the clients specified by external IDs in the request body. If enrichFromCompaniesHouse is true, enriches UK private limited companies with Companies House data.