Challenge 1: Entity, DAO, and Database — Solution

@Entity(tableName = "notes")
data class Note(
    @PrimaryKey(autoGenerate = true)
    val id: Int = 0,
    val title: String,
    val content: String
)

@Dao
interface NoteDao {
    @Query("SELECT * FROM notes ORDER BY id DESC")
    fun getAllNotes(): Flow<List<Note>>

    @Insert
    suspend fun insert(note: Note)
}

@Database(entities = [Note::class], version = 1)
abstract class AppDatabase : RoomDatabase() {
    abstract fun noteDao(): NoteDao

    companion object {
        @Volatile
        private var INSTANCE: AppDatabase? = null

        fun getDatabase(context: Context): AppDatabase {
            return INSTANCE ?: synchronized(this) {
                Room.databaseBuilder(context, AppDatabase::class.java, "app_database")
                    .build()
                    .also { INSTANCE = it }
            }
        }
    }
}

Notes:
- Note follows the same @Entity/@PrimaryKey(autoGenerate = true) shape
  as the chapter's Task example — id = 0 is a placeholder value used
  before Room assigns a real auto-generated ID on insert.
- getAllNotes() returns Flow<List<Note>>, not a plain List<Note> — this
  is what makes it "live": any successful insert() call automatically
  causes this Flow to emit a new, updated list to every collector,
  without needing to be called again manually.
- AppDatabase.getDatabase(context) is the standard entry point used to
  obtain the singleton database instance from anywhere in the app (most
  often from inside a ViewModel factory, covered in Challenge 2).