feat: add audit log system with comprehensive action tracking and user activity monitoring
Introduce AuditLog model to track system-wide user actions including authentication, course management, file operations, and user account changes. Add AuditAction enum with 17 action types (CREATE, UPDATE, DELETE, LOGIN, LOGOUT, ENROLL, UNENROLL, SUBMIT_QUIZ, APPROVE_COURSE, REJECT_COURSE, UPLOAD_FILE, DELETE_FILE, CHANGE_PASSWORD, RESET_PASSWORD, VERIFY_EMAIL, DEACTIVATE_USER, ACTIVATE_USER). Include fields
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