Decouple UserType, add Role entity and management

Removed all UserType logic and references, replacing them with a new Role model and management system. Introduced RoleManager service, repository, and API endpoint for role CRUD operations. Updated user flows and JWT claims to use RoleCode. Removed admin-only user CRUD endpoints. Updated DI, documentation, and error handling for consistency. Appsettings and DTOs now reflect the new role-based approach.

Models/Role.cs

@@ -0,0 +1,15 @@
+namespace AuthHex.Models
+{
+    public class Role
+    {
+        public Guid RoleId { get; set; }
+
+        public string Code { get; set; } = string.Empty;   // ADMIN, USER, STAFF
+        public string?Name { get; set; } = string.Empty;
+
+        public bool? IsSystemRole { get; set; }
+
+        public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
+
+    }
+}