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/digest_action_token.py

@@ -0,0 +1,44 @@
+from dataclasses import dataclass
+from models.base import BaseModel
+
+
+@dataclass
+class DigestActionToken(BaseModel):
+    user_id: str
+    child_id: str
+    entity_id: str
+    entity_type: str   # 'chore' or 'reward'
+    action: str        # 'approve' or 'deny'
+    expires_at: str    # ISO timestamp
+    used: bool = False
+    signature: str = ''
+
+    @classmethod
+    def from_dict(cls, d: dict) -> 'DigestActionToken':
+        return cls(
+            user_id=d.get('user_id'),
+            child_id=d.get('child_id'),
+            entity_id=d.get('entity_id'),
+            entity_type=d.get('entity_type'),
+            action=d.get('action'),
+            expires_at=d.get('expires_at'),
+            used=d.get('used', False),
+            signature=d.get('signature', ''),
+            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,
+            'child_id': self.child_id,
+            'entity_id': self.entity_id,
+            'entity_type': self.entity_type,
+            'action': self.action,
+            'expires_at': self.expires_at,
+            'used': self.used,
+            'signature': self.signature,
+        })
+        return base