- Add SubscriptionEntity, FeedItemEntity, SearchHistoryEntity
- Create SubscriptionDao, FeedItemDao, SearchHistoryDao with CRUD operations
- Implement RssDatabase with FTS5 virtual table for full-text search
- Add type converters for Date, String lists, and FeedItem lists
- Implement cascade delete for feed items when subscription is removed
- Add comprehensive unit tests for all DAOs
- Add database integration tests for entity round-trips and FTS
- Configure Room testing dependencies
