fix: flash_backup validation, smoke test assertions, docker/notification test coverage

- storage.py: validate initiateFlashBackup response before returning success=True
- test-tools.sh: remove set -e/inherit_errexit; add success assertion to smoke tests
- test_destructive_guards.py: add confirm-guard tests for new docker destructive actions
- test_docker.py: assert mutation variables in organizer tests; add items branch test
- test_query_validation.py: add 5 missing notification mutation schema test methods
- test_notifications.py: use lowercase importance to test uppercasing logic

Resolves review threads PRRT_kwDOO6Hdxs50FgPb PRRT_kwDOO6Hdxs50FgO4 PRRT_kwDOO6Hdxs50FgO8 PRRT_kwDOO6Hdxs50FgPI PRRT_kwDOO6Hdxs50FgPL PRRT_kwDOO6Hdxs50FgPm PRRT_kwDOO6Hdxs50E2iK PRRT_kwDOO6Hdxs50E2im

tests/test_notifications.py

@@ -327,11 +327,12 @@ class TestNewNotificationMutations:
         assert result["success"] is True
 
     async def test_unarchive_all_with_importance(self, _mock_graphql: AsyncMock) -> None:
+        """Lowercase importance input must be uppercased before being sent to GraphQL."""
         _mock_graphql.return_value = {
             "unarchiveAll": {"unread": {"total": 1}, "archive": {"total": 0}}
         }
         tool_fn = _make_tool()
-        await tool_fn(action="unarchive_all", importance="WARNING")
+        await tool_fn(action="unarchive_all", importance="warning")
         call_args = _mock_graphql.call_args
         assert call_args[0][1] == {"importance": "WARNING"}