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/serializers.py

@@ -0,0 +1,15 @@
+from rest_framework import serializers
+from .models import Notification
+
+
+class NotificationSerializer(serializers.ModelSerializer):
+    class Meta:
+        model = Notification
+        fields = [
+            'id', 'title', 'text', 'notification_type', 'action_url',
+            'is_read', 'read_at', 'created_at', 'bulk', 'send_email',
+        ]
+        read_only_fields = [
+            'id', 'title', 'text', 'notification_type', 'action_url',
+            'is_read', 'read_at', 'created_at', 'bulk', 'send_email',
+        ]