cache the subscription plans list

subscription/views.py
  - PlansListView: served from cache (subscription:plans:v1, 1h TTL);
    plans change ~never and the list is read on every subscription screen.
subscription/receivers.py (new)
  - post_save/post_delete on Plan -> cache.delete(PLANS_CACHE_KEY).
subscription/apps.py
  - SubscriptionConfig.ready() imports receivers.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

subscription/apps.py

@@ -4,3 +4,6 @@ from django.apps import AppConfig
 class SubscriptionConfig(AppConfig):
     default_auto_field = 'django.db.models.BigAutoField'
     name = 'subscription'
+
+    def ready(self):
+        from . import receivers  # noqa: F401  (باطل‌کردنِ کشِ پلن‌ها)