Explicitly import ninja

* Import ninja in ready()

Since adding the yaml parsing for feature flags into the boot process
the app crashes on startup, seemingly due module locking in Python 3.14.
debug_toolbar tries to import the package in a separate thread.

Author: gpeng

manage_breast_screening/core/apps.py

@@ -25,3 +25,6 @@ def ready(self):
         from manage_breast_screening.core.feature_flags import setup_feature_flags
 
         setup_feature_flags(_FLAGS_YAML)
+
+        # import this explicitly to avoid a crash on boot caused by a race between debug_toolbar's autoreload thread and the main one
+        import ninja  # noqa: F401