feat: Add @mention support and notification system for discussion forums

[amaydixit11]

Feature Request

Problem

The DiscussionPost system has basic like/dislike but no threaded replies visible on the post page. ReplyPost exists but there's no feature to @mention users or get notifications when someone replies.

Proposed Solution

1. Support @username mentions in DiscussionPost and ReplyPost descriptions
2. When a user is @mentioned, store a notification in the database
3. Add a /api/notifications endpoint listing mentions and replies to user's posts
4. Frontend: notification bell with unread count

Technical Notes

The DiscussionPostManage and DiscussionReplyManage classes already handle CRUD. The mention detection can use a regex pattern on the description field before saving.