Serialise segment API responses from JSON, drop rule and condition IDs

Migrate the management API read path onto `Segment.rules_data`.

We're technically breaking the API contract, but don't expect anyone to use the IDs. If we're presented with evidence to the contrary, we're going to use list indexes as IDs.

### Acceptance criteria

- Segment responses contain no `id` on rules or conditions; conditions expose `operator`, `property`, `value`, `description`.
- The segment list and retrieve endpoints no longer issue rule or condition queries for segments that have `rules_data` populated (assert query counts drop).
- For segments with no `rule_data`, list and retrieve endpoints fall back to relational data.
- OpenAPI spec and Postman collection no longer include segment rule and condition IDs.