Use explicit outbox source validation

src/govoplan_calendar/backend/outbox.py

@@ -1114,7 +1114,8 @@ def execute_calendar_outbox_operation(
         )
         return operation
 
-    assert source is not None
+    if source is None:
+        raise ValueError("Calendar outbox source is no longer available")
     operation_metadata = operation.metadata_ or {}
     overwrite = (
         bool(operation_metadata.get("overwrite"))