Model the Data: DTOs, Domain Models & Mapping

Background: Native [platform] app, [Swift / Kotlin]. Here's a sample of
the API payload(s) I receive: [paste JSON]. Here's how the app actually
uses this data in features: [describe].

Use case: Design the data models and the mapping between what the API
sends and what the domain/UI needs.

Implementation details:
- Separate DTOs (wire format) from domain models (what the app reasons
  about)—or justify collapsing them if the app is small
- iOS: Codable with custom keys/strategies for messy payloads; use value
  types and make illegal states unrepresentable. Android:
  kotlinx.serialization / Moshi with data classes.
- Handle nullability, optional fields, dates, enums with unknown values,
  and snake_case → camelCase
- Where mapping happens (DTO → domain) and how mapping errors are handled
- Keep the domain model free of API quirks

Deliverable: The DTO definitions for the payload, the clean domain
models, the mapping functions, the decoding strategy for the messy
fields, and how a malformed payload is handled gracefully.