Add default website URL for tests

backend/src/main/java/com/openisle/service/NotificationService.java

@@ -32,7 +32,7 @@ public class NotificationService {
     private final ReactionRepository reactionRepository;
     private final Executor notificationExecutor;
 
-    @Value("${app.website-url}")
+    @Value("${app.website-url:https://www.open-isle.com}")
     private String websiteUrl;
 
     private static final Pattern MENTION_PATTERN = Pattern.compile("@\\[([^\\]]+)\\]");