Evaluate segments from the stored JSON

[khvn26]

Migrate Core API flag evaluation logic onto Segment.rules_data.

Scope

1. Rewrite map_segment_to_engine and map_segment_rule_to_engine to build engine models from Segment.rules_data instead of segment.rules.all() / rule.conditions.all().
2. Remove the rules / rules__conditions / rules__rules__conditions prefetch chains from the evaluation read path:

• projects/services.py.
• environments/managers.py.
• environments/models.py.
• environments/identities/models.py.

Acceptance criteria

• Identity segment evaluation (Identity.get_segments) returns the same matching segments before and after the change.
• The DynamoDB environment document and the compressed environment document contain identical segment data to the pre-change output.
• The project and environment segment caches feeding the mappers no longer issue rule or condition queries (assert query counts drop).