Backend: Add _growthbook attribute to CouchersContext

Declare _growthbook directly on CouchersContext so _get_growthbook no
longer needs type: ignore comments.

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

Author: aapeliv

app/backend/src/couchers/context.py

@@ -1,9 +1,12 @@
-from typing import NoReturn, cast
+from typing import TYPE_CHECKING, NoReturn, cast
 
 import grpc
 
 from couchers.i18n import LocalizationContext
 
+if TYPE_CHECKING:
+    from growthbook import GrowthBook
+
 
 class NonInteractiveContextException(Exception):
     """If this exception is raised, it is a programming error"""
@@ -79,6 +82,7 @@ def __init__(
         self.__is_interactive = is_interactive
         self.__logged_in = self._user_id is not None
         self.__cookies: list[str] = []
+        self._growthbook: GrowthBook | None = None
 
         if self.__is_interactive:
             if not self._grpc_context:

app/backend/src/couchers/experimentation.py

@@ -153,7 +153,8 @@ def _get_growthbook(context: CouchersContext) -> GrowthBook:
     `client_key` keeps the GrowthBook a pure evaluator: no callback
     registration on the library's process-wide singleton.
     """
-    if not hasattr(context, "_growthbook"):
+    gb = context._growthbook
+    if gb is None:
         with _state_lock:
             features = _state["features"]
             saved_groups = _state["savedGroups"]
@@ -169,8 +170,8 @@ def on_experiment_viewed(experiment: Experiment, result: Result, **kwargs: Any)
             savedGroups=saved_groups,
             on_experiment_viewed=on_experiment_viewed,
         )
-        context._growthbook = gb  # type: ignore[attr-defined]
-    return context._growthbook  # type: ignore[attr-defined, no-any-return]
+        context._growthbook = gb
+    return gb
 
 
 def check_gate(context: CouchersContext, gate_name: str) -> bool: