Design a Repository Layer

Background: Native [platform] app, [SwiftUI+Combine / Compose]. I have
these data sources: [e.g., REST API, local database, in-memory cache,
key-value store]. The domain works with these models: [list].

Use case: Design a repository layer that hides where data comes from, so
the rest of the app asks for data without knowing it's remote, cached,
or local.

Implementation details:
- A repository interface per domain area, exposing domain models (not
  raw DTOs)
- A single-source-of-truth / offline-first strategy: serve cache, refresh
  from network, emit updates
- iOS: expose Combine publishers (or async/await) to callers
- Android: expose Kotlin Flow (or suspend functions) to callers
- Where DTO-to-domain mapping happens, and where errors are translated

Layer & dependencies: Repository lives in the data layer behind a
domain-defined interface; the domain/UI never imports networking or
persistence types directly.

Deliverable: The repository interface, a concrete implementation
coordinating network + local sources with the offline-first flow, the
mapping layer, and a note on how view models consume it.