Add ELM (Leave) module integration

[AnKiT-V01]

This PR integrates the ELM (Employee Leave Management) module into Fusion-Integrated for Assignment 6.

Changes Included

• Module Skeleton: Django app, models, serializers, views, and migrations for leave requests and transitions.
• Domain Logic: Pure Python state machine and entitlement logic (CL, RH, EL credits).
• Integration & Wiring: Base settings registration, API URLs mount, import linter layers, and permission manifest generation.
• Verification: Verified zero cross-module foreign keys, plural contract getters, nav/route synchronization, manifest validation, and passing domain test suite.