Stop using the SegmentRule and Condition models

Migrate the last segment rule/condition readers (release pipeline, organisation export) onto `Segment.rules_data` and stop writing rows. Leave the models so historical segments can still be read, and mark them as deprecated.

### Acceptance criteria

- `SegmentRule` and `Condition` are clearly marked as deprecated.
- Creating or updating a segment writes no `SegmentRule` or `Condition` rows.
- The release-pipeline phased-rollout percentage is read correctly from `rules_data`.
- An organisation import produces `rules_data` and no `SegmentRule` and `Condition` rows.
- The OpenAPI schema does not rely on the `SegmentRule` model.
- The ordering settings are gone and no test depends on them.
- The newly introduced `Segment.rules_data` is renamed to `Segment.rules` — rules naturally flow from the serialiser into the model.