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.

frontend/vue-app/src/common/models.ts

@@ -63,6 +63,8 @@ export interface User {
   deletion_in_progress: boolean
   deletion_attempted_at: string | null
   role: string
+  timezone: string | null
+  email_digest_enabled: boolean
 }
 
 export interface Child {