Backend changes for Segment redesign

[amurphy-cl]

Make backend changes to support Segment design refresh

Old segment structure doc: https://docs.google.com/document/d/1IxFuCt_UGGyq9zynPTO9ubhWyKk5vo7UYupPIzNJEKI/edit?tab=t.0#heading=h.k7a8qo328l3s

Changes from old to new
For new segments: we won't have the three categories e.g. individual, group, and sub-segment. Instead we will only have sub-segments.

• New endpoint for new list - This will create a new private segment and add it to the subSegments array of the parent public segment. The new private segment will have a defined 'listType' attribute. It should be a segment edit with the private subsegment creation as part of the transaction
• New endpoint to delete a list - delete the record for the list and remove it from the parent segment's 'subsegments` array
• Database schema migration to add an optional 'listType' string attribute.

NB: creating/deleting/editing a new segment should be able to use the existing endpoints.