Define ContactChapter domain model and mapper

Check whether the project uses `freezed` for data classes — if so, annotate `ContactChapter` with `@freezed` and generate via `build_runner` for `copyWith`, `==`, `hashCode`, and `toString` for free. If not, implement manually and keep it consistent with other domain models. For DateTime parsing, use `DateTime.parse(json['joined_at'] as String)` and wrap in a try/catch that throws a domain-level `MappingException` with context rather than a raw `FormatException`. Use `_` prefixed private constructor pattern if `freezed` is not in use, exposing a factory constructor `ContactChapter({...})` for clarity.

Snake_case to camelCase key mapping must be explicit in the mapper — do not rely on `json_serializable` auto-naming unless it is already the project standard.

Unit tests in `test/domain/models/contact_chapter_test.dart` and `test/data/mappers/contact_chapter_mapper_test.dart`. Cover: (1) `fromJson` with a complete valid JSON map returns a correctly populated `ContactChapter`; (2) `fromJson` with `role_in_chapter: null` returns a `ContactChapter` with `roleInChapter == null`; (3) `toInsertJson` does not include `created_at` or `updated_at` keys; (4) two `ContactChapter` instances with the same `contactId` and `chapterId` are equal (`==` returns true). Aim for 100% line coverage of both files.