Use HassKey in backup (#127546)

* Use HassKey in backup

* Use DATA_MANAGER

homeassistant/components/backup/const.py

@@ -1,8 +1,17 @@
 """Constants for the Backup integration."""
 
+from __future__ import annotations
+
 from logging import getLogger
+from typing import TYPE_CHECKING
+
+from homeassistant.util.hass_dict import HassKey
+
+if TYPE_CHECKING:
+    from .manager import BackupManager
 
 DOMAIN = "backup"
+DATA_MANAGER: HassKey[BackupManager] = HassKey(DOMAIN)
 LOGGER = getLogger(__package__)
 
 EXCLUDE_FROM_BACKUP = [