Implement Project Subscriptions & Subscription Frontend UI

## Project Subscription Notifications & Management UI
Part of: #3227

### Background
Snapshot Notifications: Implemented in Core Notification Infrastructure & Snapshot Triggers #3637

Chapter/Event Subscriptions: Implemented in Implement Entity-Level Subscriptions for Chapters and Events #3900

Project Subscriptions: Not yet implemented

### Notification Rules
#### Projects
All Projects Subscription

When a new release is published for a project, send an email notification to all users who subscribed to "all projects."

When a milestone is updated or closed for a project, send an email notification to all users who subscribed to "all projects."

Specific Project Subscription

When a specific project has a new release/milestone, send an email notification only to users who subscribed to that particular project.

### Frontend User Interface
Users can follow/subscribe to specific: Chapter, Project, Event

Users can subscribe to: All Chapters, All Projects, All Events, All Snapshots

Users can view and manage their subscriptions in Profile Settings

### Tasks
#### Project Notifications (Backend)
[ ] Implement signal handler on Project model post_save.

[ ] Detect new release and milestone changes.

[ ] Query subscribers based on trigger type.

[ ] Queue notifications to Redis Stream.

#### Frontend Implementation
[ ] Add subscription UI in user profile.

[ ] Allow users to subscribe/unsubscribe to entities (Chapter, Project, Event, Snapshot).

[ ] Show subscription status for each entity type.

#### Worker Updates & Testing
[ ] Support both global and specific entity subscriptions in worker logic.

[ ] Write unit tests for subscription triggers and UI state.

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

- [x] Yes
- [ ] No