Introduce notifications app and integrate emails

Add a new notifications app (models, serializers, views, admin, tasks, consumers, routing, urls, tests) with Notification.notify for in-app websocket pushes and optional email delivery. Centralize email sending in notifications.tasks.send_email_with_context and re-export it from account.tasks for compatibility. Update account and commerce and advertisement tasks to use Notification.notify/_notify_order and the new email helper; adjust order/notification tasks to consolidate logic. Wire notifications into ASGI routing, settings and URL conf. Misc: handle OSError when importing weasyprint, add tmp/ to .gitignore, add local .claude PowerShell checks, add social.blog skeleton, and remove legacy ews-component test files.

backend/notifications/urls.py

@@ -0,0 +1,10 @@
+from django.urls import path, include
+from rest_framework.routers import DefaultRouter
+from . import views
+
+router = DefaultRouter()
+router.register(r'', views.NotificationViewSet, basename='notification')
+
+urlpatterns = [
+    path('', include(router.urls)),
+]