Check password early on backup restore (#5519)

Introduce a validate password method which only peaks into the archive
to validate the password before starting the actual restore process.
This makes sure that a wrong password returns an error even when
restoring the backup in background.

tests/api/test_backups.py

@@ -478,7 +478,7 @@ async def test_restore_immediate_errors(
 
     with (
         patch.object(Backup, "protected", new=PropertyMock(return_value=True)),
-        patch.object(Backup, "set_password", return_value=False),
+        patch.object(Backup, "validate_password", return_value=False),
     ):
         resp = await api_client.post(
             f"/backups/{mock_partial_backup.slug}/restore/partial",