Add comprehensive audit logging system

[Alen983]

Track all important actions (user creation, leave approvals, policy changes) for compliance and debugging.
Relevant part of the codebase: Create new backend/app/models.py - AuditLog model, and add logging to critical operations
Expected outcome:

• Create AuditLog model with fields: user_id, action, resource_type, resource_id, timestamp, ip_address, details
• Add audit logging decorator or middleware
• Log all admin actions, leave approvals/rejections, user updates
• Add admin endpoint to query audit logs
• Consider log retention policies