Email Implement notification & subscription system for Snapshots and entities

Implement an email-based notification system to keep users informed about:
1. Snapshot publications - Alert when new Snapshot is published
2. Entity-level subscriptions - Users can follow specific Projects, Chapters, and Events and receive notifications for relevant updates
Scope (Email-only MVP):
Snapshot Notifications:
- Send email to Snapshot subscribers when new Snapshot is published
Entity Subscriptions:
- Users can subscribe to specific OWASP Projects, Chapters, and Events
- Notification triggers:
- New event added to subscribed Chapter → Email to Chapter subscribers
- Event deadline approaching (X days before) → Email to event subscribers
- New release published for Project → Email to Project subscribers
- Milestone updates/closures for Project → Email to Project subscribers

### Sub-Issues
- [ ] #3637
- [ ] #3900
- [ ] #4145

**Are you going to work on implementing this?**

- [x] Yes
- [ ] No