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/vontor_cz/urls.py

@@ -43,6 +43,7 @@ urlpatterns = [
     path('api/configuration/', include('configuration.urls')),
     path('api/advertisement/', include('advertisement.urls')),
 
+    path('api/notifications/', include('notifications.urls')),
     path('api/social/hubs/', include('social.hubs.urls')),
     path('api/social/posts/', include('social.posts.urls')),
     path('api/social/', include('social.chat.urls')),