Define domain models for admin data layer

Follow the existing domain model pattern in the codebase. Use final fields with a const constructor where possible. For fromJson, use the pattern: field = json['field_name'] as Type — add explicit casts and null checks. For DateTime parsing, use DateTime.parse(json['created_at'] as String) and .toLocal() if the UI displays local times.

For the OrgSubtree.contains() helper, implement as bool contains(String orgId) => subtreeOrganisationIds.contains(orgId) || orgId == rootOrganisationId. The AdminRole and ReimbursementStatus enums should have a fromString(String) factory for JSON deserialization using a switch expression. Avoid using code generation (json_serializable, freezed) unless the project already uses it — adding new build_runner dependencies mid-project adds friction. If the project does use freezed, use it consistently.

Place enums in a separate file lib/domain/admin/admin_enums.dart to keep model files focused.

Unit tests only — no widget or integration tests needed for pure domain models. Test file: test/domain/admin/admin_models_test.dart. Required test groups: (1) Serialization — fromJson produces correct field values from a fixture JSON map for each model; toJson produces the correct snake_case map; fromJson(toJson(model)) equals original model; (2) Equality — two models with identical fields are equal; models differing in one field are not equal; (3) CopyWith — copyWith with no arguments returns equal model; copyWith with one argument changes only that field; copyWith supports null for nullable fields; (4) Edge cases — fromJson handles null optional fields without throwing; DateTime fields parse ISO 8601 strings correctly; enum fields throw on invalid values. Use const fixture maps defined at the top of the test file.