Notify Users About Replies to Their Comments in Budget Proposals

## Description
Introduce a way to **notify users** when someone replies to their **comments** in the **Budget Proposals** section.
The goal is to make it easier for users to stay informed about discussions without requiring them to manually reopen proposals to check for replies.
This task leaves flexibility on how the notification mechanism will be implemented (email, in-app, or other), allowing further discussion and decision-making.

## Acceptance Criteria
- [ ] Users are **notified** when a reply is made to their comment.
- [ ] Notifications are clear and link back to the relevant discussion.