Admin Create Org Rekognition Collection

Create a new Rekognition collection for this organization and save its ID to the DB. Auto-generates the collection ID as 'pavoot-{env}-{org-name-slug}' where env comes from the request body (dev/staging/prod, detected client-side from the browser hostname). Returns an error if the org already has a custom collection set. Migrates existing faces from the org's projects into the new collection as a background task.