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

[khvn26]

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.