Comment logged-out anonymous evaluation in _get_growthbook

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: aapeliv

app/backend/src/couchers/context.py

@@ -207,6 +207,7 @@ def _get_feature_value[T](self, flag_key: str, default: T) -> T:
 
     def _get_growthbook(self) -> GrowthBook:
         if self._growthbook is None:
+            # _user_id is None when logged out: evaluate anonymously, falling through to defaults.
             self._growthbook = experimentation._create_evaluator(self._user_id)
         return self._growthbook