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/db/push_subscriptions.py

@@ -0,0 +1,38 @@
+from tinydb import Query
+from db.db import push_subscriptions_db
+from models.push_subscription import PushSubscription
+
+
+def get_subscriptions_by_user(user_id: str) -> list[PushSubscription]:
+    """Return all push subscriptions for a user."""
+    Q = Query()
+    results = push_subscriptions_db.search(Q.user_id == user_id)
+    return [PushSubscription.from_dict(r) for r in results]
+
+
+def upsert_subscription(user_id: str, endpoint: str, keys: dict) -> PushSubscription:
+    """Insert or update a subscription for the given user+endpoint pair."""
+    Q = Query()
+    existing = push_subscriptions_db.get((Q.user_id == user_id) & (Q.endpoint == endpoint))
+    if existing:
+        sub = PushSubscription.from_dict(existing)
+        sub.keys = keys
+        sub.touch()
+        push_subscriptions_db.update(sub.to_dict(), (Q.user_id == user_id) & (Q.endpoint == endpoint))
+        return sub
+    sub = PushSubscription(user_id=user_id, endpoint=endpoint, keys=keys)
+    push_subscriptions_db.insert(sub.to_dict())
+    return sub
+
+
+def delete_by_endpoint(user_id: str, endpoint: str) -> int:
+    """Remove the subscription with the given endpoint for this user. Returns count removed."""
+    Q = Query()
+    removed = push_subscriptions_db.remove((Q.user_id == user_id) & (Q.endpoint == endpoint))
+    return len(removed)
+
+
+def delete_subscription_by_id(subscription_id: str) -> None:
+    """Remove a subscription by its ID (used when push delivery fails)."""
+    Q = Query()
+    push_subscriptions_db.remove(Q.id == subscription_id)