Notifications from WorkManager

  private suspend fun cachedBooks(): List<Book> = repo.observeBooks().firstOrNull().orEmpty()
  /** 5.30.3 and 5.30.5 announce price drops, redraw the widget, return the book count. */
  private suspend fun afterSync(before: List<Book>): Int {
    val oldPrice = before.associate { it.id to it.price }
    val books = cachedBooks()
    books.filter { it.price < (oldPrice[it.id] ?: it.price) }.forEach {
      BookNestNotifications.showPriceDrop(applicationContext, it, oldPrice.getValue(it.id))
    }
    BookNestWidget().updateAll(applicationContext)  // every placed BookNest widget
    return books.size
  }