Basic domain model

[pablonete]

Part of Epic 0: Navigable demo.

Define the first version of the domain model: kids, activities from GitHub issues, and completed activities.

Decisions

• Minimum fields for kids and activities.
• How activities map to GitHub issues.
• How completed activities are represented before persistence.
• Centralize data access so even when it's mocked, we can easily replace to fetching from backend once we implement it