feat: implement backend notification system for comment mentions

[Arcadi4]

Summary

Implements backend-only in-app notification MVP for issue #61, focused on comment mention notifications.

What Changed

New notifications app (apps/backend/notifications/):

• Notification model with UUID, timestamps, recipient FK, actor/target/action_object GFKs, notification_type, verb, is_read, data, dedupe_key
• Service layer with idempotent notification creation and read state management
• REST API at /v1/notifications/ with list, retrieve, unread_count, mark_read, mark_all_read
• Admin registration with filters and search

Comment mention integration (apps/backend/comments/services.py):

• create_comment() creates mention notifications after comment creation
• update_comment() creates notifications only for newly added mentions
• Self-mentions filtered, duplicate notifications prevented via dedupe_key

Registration updates:

• Added to INSTALLED_APPS in base.py and test.py
• Registered in backend URLs at /v1/
• Updated backend-lint.yml workflow
• Updated README.md and AGENTS.md

API Endpoints

• GET /v1/notifications/ - List user's notifications (filterable by is_read, notification_type)
• GET /v1/notifications/{id}/ - Retrieve single notification
• GET /v1/notifications/unread_count/ - Get unread count
• POST /v1/notifications/{id}/mark_read/ - Mark one as read
• POST /v1/notifications/mark_all_read/ - Mark all as read

All endpoints require authentication and are scoped to request.user.

Closes #61