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/views.py

@@ -1,3 +1,4 @@
+from django.core.cache import cache
 from django.db import transaction
 from django.db.models import F
 from django.shortcuts import get_object_or_404
@@ -38,26 +39,34 @@ class GuestAwarePurchaseThrottle(ScopedRateThrottle):
         return self.cache_format % {'scope': self.scope, 'ident': ident}
 
 
+# پلن‌ها تقریباً هیچ‌وقت عوض نمی‌شوند و روی صفحه‌ی اشتراک هر بار خوانده می‌شوند.
+# با هر save/delete روی Plan کش باطل می‌شود (subscription/receivers.py).
+PLANS_CACHE_KEY = 'subscription:plans:v1'
+PLANS_CACHE_TTL = 3600
+
+
 class PlansListView(APIView):
     """GET /api/subscription/plans — list available plans."""
 
     @extend_schema(responses=PlansListResponseSerializer, tags=['Subscription (subscription)'])
     def get(self, request):
-        plans = Plan.objects.all()
-        data = {
-            'plans': [
-                {
-                    'id': str(p.id),
-                    'title': p.title,
-                    'price': p.price,
-                    'currency': p.currency,
-                    'popular': p.popular,
-                    'discountPercent': p.discount_percent,
-                    'durationDays': p.duration_days,
-                }
-                for p in plans
-            ]
-        }
+        def build():
+            return {
+                'plans': [
+                    {
+                        'id': str(p.id),
+                        'title': p.title,
+                        'price': p.price,
+                        'currency': p.currency,
+                        'popular': p.popular,
+                        'discountPercent': p.discount_percent,
+                        'durationDays': p.duration_days,
+                    }
+                    for p in Plan.objects.all()
+                ]
+            }
+
+        data = cache.get_or_set(PLANS_CACHE_KEY, build, PLANS_CACHE_TTL)
         return Response(PlansListResponseSerializer(data).data)