Add employee leave management module

[Arnv-Xd]

Description

Adds the Employee Leave Management (ELM) module with a Django data model and REST API endpoints for managing leave requests.

Changes Made

• Added the Leave model with leave categories, request status, start and end dates, reason, and timestamps.
• Added a REST Framework serializer for leave requests.
• Implemented a LeaveViewSet for CRUD operations on leave requests.
• Configured API routing for leave endpoints under /api/v1/elm/.
• Added module registration metadata, navigation items, permissions, and role grants for staff, HR, and admin.
• Added application configuration and model package initialization.

Notes

• Follows the existing modular Django application structure.
• Automated test verification remains pending.