Add unique index for user_ldap group memberships

Signed-off-by: Côme Chilliet <come.chilliet@nextcloud.com>

apps/user_ldap/lib/Migration/Version1190Date20230706134108.php

@@ -63,6 +63,7 @@ class Version1190Date20230706134108 extends SimpleMigrationStep {
 				'default' => '',
 			]);
 			$table->setPrimaryKey(['id']);
+			$table->addUniqueIndex(['groupid', 'userid'], 'user_ldap_membership_unique');
 			return $schema;
 		} else {
 			return null;