Add push notification functionality with tests and digest scheduler

- Implemented push subscription API with tests for subscribing and unsubscribing users.
- Created web push notification tests triggered by child actions.
- Added digest scheduler to send email digests to users at 9 PM local time.
- Developed utility functions for creating and validating digest action tokens.
- Integrated web push sender to handle sending notifications to users.
- Added service worker for handling push notifications in the frontend.
- Created a push opt-in component for user notification preferences.
- Implemented tests for the push opt-in component to ensure correct behavior.
- Updated frontend services to manage push subscriptions and permissions.

backend/tests/conftest.py

@@ -2,6 +2,9 @@ import os
 os.environ['DB_ENV'] = 'test'
 os.environ.setdefault('SECRET_KEY', 'test-secret-key')
 os.environ.setdefault('REFRESH_TOKEN_EXPIRY_DAYS', '90')
+os.environ.setdefault('DIGEST_TOKEN_SECRET', 'test-digest-secret')
+os.environ.setdefault('VAPID_PUBLIC_KEY', 'test-vapid-public-key')
+os.environ.setdefault('VAPID_PRIVATE_KEY', 'test-vapid-private-key')
 import sys
 import pytest