Add ninja to INSTALLED_APPS

* explicitly add ninja

We used this for the API but it is also used by django-debug-toolbar
which loads it in a separate autoload thread. This causes a race
condition and as of Python 3.14 this can raise an error and stop the
app booting if the toolbar is enabled.

Adding the ninja app here also fixes a current issue where the docs
assets aren't being loaded from the CDN. This is outlined in the Ninja
docs https://django-ninja.dev/guides/api-docs/

Author: gpeng

manage_breast_screening/config/settings/base.py

@@ -73,6 +73,7 @@ def list_env(key):
     "manage_breast_screening.reading",
     "rules.apps.AutodiscoverRulesConfig",
     "csp",
+    "ninja",
 ]
 
 if DJANGO_ENV != "production":