Stop using the SegmentRule and Condition models

[khvn26]

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.