manage MailerLite groups and read segment membership via the API

Steps

1. Create a group with POST https://connect.mailerlite.com/api/groups (name only, max 255 chars)
2. Assign/unassign subscribers to a group with POST/DELETE /api/subscribers/{subscriber_id}/groups/{group_id}
3. Bulk-import subscribers directly into a group with POST /api/groups/{group_id}/import-subscribers (async, 5 requests/minute limit)
4. List segments with GET /api/segments and fetch their members with GET /api/segments/{segment_id}/subscribers
5. Rename or delete a segment with PUT/DELETE /api/segments/{segment_id}