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/commerce/models.py

@@ -11,7 +11,7 @@ from django.core.validators import MaxValueValidator, MinValueValidator, validat
 
 try:
     from weasyprint import HTML
-except ImportError:
+except (ImportError, OSError):
     HTML = None
 
 import os