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

[khvn26]

Migrate the management API read path onto Segment.rules_data.

Scope

1. Serialise segment API responses from Segment.rules_data instead of the nested rule and condition row serializers.
2. Drop rule and condition id from responses.
3. Remove the rules / rules__conditions / rules__rules__conditions prefetch chain from segments/views.py.
4. Make sure the OpenAPI spec no longer includes rule and condition 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 (assert query counts drop).
• OpenAPI spec and Postman collection no longer include segment rule and condition IDs.