Design the Persistence Layer

Background: Native [iOS / Android] app, [SwiftUI / Compose]. I need to
store this locally: [describe entities and relationships]. Access
patterns: [main reads/writes and queries]. Roughly [data volume] and
[grows how fast].

Use case: Design the local persistence layer and choose the storage tech.

Implementation details:
- iOS: compare SwiftData, Core Data, and (for simple cases) lighter
  options; recommend one for my target iOS version and complexity
- Android: compare Room and lighter options (DataStore for key-value);
  recommend one
- Define the entities/models, relationships, and the indexes the access
  patterns need
- Where the persistence layer exposes results reactively (Combine
  publishers / Flow) so the UI updates on change
- Threading: writes off the main thread, observed reads on it

Layer & dependencies: Persistence sits behind the repository; the domain
works with domain models, not persistence entities.

Deliverable: A storage recommendation with reasoning, the schema/model
definitions, the key queries, the reactive read setup, and the threading
rules to follow.