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/models/push_subscription.py

@@ -0,0 +1,29 @@
+from dataclasses import dataclass
+from models.base import BaseModel
+
+
+@dataclass
+class PushSubscription(BaseModel):
+    user_id: str
+    endpoint: str
+    keys: dict  # {'p256dh': str, 'auth': str}
+
+    @classmethod
+    def from_dict(cls, d: dict) -> 'PushSubscription':
+        return cls(
+            user_id=d.get('user_id'),
+            endpoint=d.get('endpoint'),
+            keys=d.get('keys', {}),
+            id=d.get('id'),
+            created_at=d.get('created_at'),
+            updated_at=d.get('updated_at'),
+        )
+
+    def to_dict(self) -> dict:
+        base = super().to_dict()
+        base.update({
+            'user_id': self.user_id,
+            'endpoint': self.endpoint,
+            'keys': self.keys,
+        })
+        return base