Draft: implementation of notification using user activity logging

Title

feat(backend): implement user activity tracking foundation for notification system

Description

This merge request introduces the initial implementation of the notification architecture for the Corpus Backend by establishing a structured user activity tracking system.

The goal of this change is to enable a scalable three-step notification workflow:

1. User Activity Logging
2. In-App Notifications
3. Push Notifications

Things to implement

User Activity Logging System

• Added a dedicated database table for storing user activity logs
• Introduced structured activity types for consistent event classification
• Implemented actor → recipient relationship tracking
• Stored timestamps and relevant metadata for each activity
• Integrated activity logging into backend service-level operations so events are recorded automatically during supported actions

Architecture Notes

The implementation follows clean architecture principles:

• Backend-driven event generation
• Dedicated activity log model
• Service-layer integration
• Modular design supporting future notification extensions
• Optimized schema prepared for indexing and scalable querying

This change lays the foundation required for a modern interaction-aware user experience similar to contemporary social platforms.

Checklist

• The feature has been fully implemented.
• Tests for the new feature are included and passing.
• User documentation/guides have been updated (if applicable).
• Impact on existing functionality has been considered.

Related Issue(s)

Closes #97 (closed)