Add Notification Trigger for Top-Ranked Recommendations

**Is your feature request related to a problem? Please describe.**
I’m looking for a way to proactively notify users when the most relevant recommendations are available to them. Currently, Gorse provides excellent recommendations, but there’s no built-in mechanism to alert users about high-priority or top-ranked content in real time.

Right now, I am using cron jobs to fetch the highest-ranked post for each user on an hourly basis and send notifications manually. While this works, it’s not efficient for real-time notifications and introduces delays, which can impact engagement, especially for time-sensitive content like trending posts or limited-time offers.

**Describe the solution you'd like**
I’d like to see a feature that triggers a notification when a post or item for a user reaches the highest rank in their recommendation list.

Key Aspects:
Triggering Logic:

Automatically detect when an item achieves the top rank (or top N ranks, configurable) for a user in their recommendation list.
Event/Notification Integration:

Emit events or provide webhook support to integrate with external notification systems, such as Firebase, Twilio, or email services.
Customization Options:

Allow developers to configure the trigger thresholds (e.g., top 1, top 3, etc.) or enable/disable this feature per use case.

**Describe alternatives you've considered**
Currently, I’m using cron jobs to fetch the highest-ranked post for each user on an hourly basis and send notifications. While this approach works, it’s not efficient for real-time updates. The delay introduced by the hourly schedule can reduce engagement, especially for time-sensitive content, and maintaining the cron job scripts adds extra complexity and overhead.

**Additional context**
This feature would be highly valuable for applications where user engagement is tied to delivering timely, relevant content. For example:

Social Media Applications: Notify users when trending posts or new content become highly relevant to them.
E-commerce Platforms: Alert users about the top-ranked products they might be interested in.
News Aggregators: Push notifications for breaking news articles that are most relevant to a user.

My English is not perfect, so I got some help from Chatgpt to write my feature request.