Behavior: Initiates a batch action to migrate assets between storage platforms using the provided bucket replacement mappings. The platform setup must be completed before calling this endpoint. This endpoint only triggers the migration; the actual processing occurs asynchronously.

Authorization: No [Authorize] attribute, but the user must be an admin (checked via IsAdminUser).

Side Effects: Queues a batch migration job that moves assets between storage buckets. This is a long-running asynchronous operation.

Error Scenarios: Returns 400 if replaceBuckets is null or empty, or if the user is not an admin. Throws a VibrantException for permission or validation failures.