feat: implement admin role management and access control

[CollinsC1O]

Implement Admin Role Management and Access Control

Closes #23

Overview

This PR implements a comprehensive admin role management system that provides secure access control for all privileged contract operations including verification approvals, treasury management, and fee configuration.

What's Changed

Core Implementation

• ✅ Admin registry with add/remove functionality
• ✅ Access control for verification approvals/rejections
• ✅ Access control for fee and treasury management
• ✅ Event emission for all admin role changes
• ✅ Protection against edge cases (last admin removal, double initialization)

New Files

• src/admin_registry.rs - Admin management logic
• src/admin_tests.rs - Unit tests
• src/integration_tests.rs - Integration tests

Modified Files

• src/lib.rs - Added 9 public admin functions
• src/events.rs - Added AdminAdded, AdminRemoved, TreasuryUpdated events
• src/types.rs - Added admin storage keys
• src/verification_registry.rs - Integrated admin authorization
• src/fee_manager.rs - Integrated admin authorization

Test:

All test both old and newly implemented, passes successfully

Contract fully compiles

Key Features

Admin Management

initialize(admin: Address)                    // One-time initialization
add_admin(caller: Address, new_admin: Address)    // Add new admin
remove_admin(caller: Address, admin: Address)     // Remove admin
is_admin(address: Address) -> bool                // Check admin status
list_admins() -> Vec<Address>                     // List all admins