Add initial Employee Leave Management backend scaffold

[ansika-patti]

Summary

• Register the ELM module in Django settings and API URL configuration.
• Add LeaveType and LeaveRequest models.
• Add serializers and REST API viewsets for both models.
• Register the models in Django admin and generate the initial migration for the updated models.

Verification

• Python syntax compilation passed for the updated files.
• python manage.py check reported no issues.
• Django generated the migration successfully.
• PostgreSQL credentials were unavailable, so database migration execution was not verified.

Scope and limitations

This is an initial backend scaffold. Full ELM workflows and business-rule compliance have not been verified.