Fix registration API to use new group-based user creation method

- Replace deprecated createUserWithRoleRegistration with createUserWithGroupAssignment
- Update comments to reflect group-based system instead of role-based
- Resolves registration error when creating new users

server/api/registration.post.ts

@@ -92,8 +92,8 @@ export default defineEventHandler(async (event) => {
       });
     }
 
-    // 5. Create Keycloak user with role-based registration
-    console.log('[api/registration.post] Creating Keycloak user with role-based system...');
+    // 5. Create Keycloak user with group-based assignment
+    console.log('[api/registration.post] Creating Keycloak user with group-based system...');
     const paymentDueDate = new Date();
     paymentDueDate.setMonth(paymentDueDate.getMonth() + 1); // 1 month from registration
     
@@ -108,11 +108,11 @@ export default defineEventHandler(async (event) => {
       membershipTier: 'user' as const
     };
 
-    createdKeycloakId = await keycloakAdmin.createUserWithRoleRegistration({
+    createdKeycloakId = await keycloakAdmin.createUserWithGroupAssignment({
       email: body.email,
       firstName: body.first_name,
       lastName: body.last_name,
-      membershipTier: 'user', // All public registrations default to 'user' role
+      membershipTier: 'user', // All public registrations default to 'user' group
       membershipData
     });