feat(rbac): implement hierarchical permission-based RBAC with dynamic role assignment and audit logging (#50)

[devxsameer]

Description

This PR implements a hierarchical, permission-based Role-Based Access Control (RBAC) system aligned with Issue #50 requirements. It introduces the VetStaff role, expands the permission model, enforces permission-first route guards, enables dynamic role assignment, and integrates a complete audit trail for role changes. The system ensures secure, extensible, and maintainable authorization across the backend.

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Related Issues

Closes #50

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Changes Made

• Added VetStaff role to the system
• Added missing permissions (SHARE_RECORDS, PRESCRIBE, READ_ASSIGNED_PETS, UPDATE_APPOINTMENTS, CREATE_NOTES)
• Implemented recursive role-permission resolution for hierarchy support
• Refactored RolesGuard to enforce permission-first authorization
• Strongly typed @Permissions() decorator using Permission[]
• Added admin-protected endpoints:
  • POST /roles/assign
  • POST /roles/remove
  • GET /roles/user/:userId
• Implemented soft role removal (isActive = false)
• Integrated RoleAuditLog for role assignment and removal
• Updated RolesPermissionsSeeder to align with required permission mappings
• Ensured active role filtering during permission resolution