feat: cache user favorites to local database

[arkon]

Introduces a two-table Room database (songs + favourites) so the app can
show cached favourites instantly on launch while refreshing in the background.

• songs table stores song metadata; structured to later hold all songs
• favourites table has a FK to songs and is cleared when a different user
  logs in (detected via uuid stored in SharedPreferences)
• UserService now reads from the DB cache before the network response
  arrives, then replaces the cache with fresh data
• Favourite toggles keep the DB in sync via the existing favoriteEvents flow

https://claude.ai/code/session_011KtuHPsAhM395DnPGr6Zcs

[arkon]

Code review

Found 1 issue:

1. @Transaction on a concrete suspend fun in a Kotlin DAO interface is not honored by Room's annotation processor — replaceAll is not atomic. Room only generates a transaction wrapper for abstract methods or methods in abstract classes; it does not wrap concrete default interface methods. As a result, deleteAll() and insertAll() each run in their own transaction, leaving a window where the favourites table is empty. A concurrent read (e.g., from the favoriteEvents collector) could observe zero favourites during this gap. Fix by changing FavouritesDao from an interface to an abstract class, or by using withTransaction { } from room-ktx.

android-app/app/src/main/kotlin/me/echeung/moemoekyun/data/database/dao/FavouritesDao.kt

Lines 34 to 41 in 7d38b4e

	suspend fun deleteAll()
	@Transaction
	suspend fun replaceAll(favourites: List<FavouriteEntity>) {
	deleteAll()
	insertAll(favourites)
	}
	}

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