Streamline segment mutations in the frontend

Move the normal segment edit flow to full-replace semantics. Drop the `delete: true` convention both from frontend and backend.

### Acceptance criteria

- Removing a rule or condition in the editor omits it from the request body; no entry carries `delete: true`.
- `SegmentRule` and `SegmentCondition` types no longer carry `id`.
- Editing a segment work against the id-free API.
- API does not expect `delete: true` anymore.
- The change request segment diff renders unchanged.