feat: add logging for successful user sign-up in SignUpService

Author: polyglot-k

src/main/java/life/mosu/mosuserver/application/auth/SignUpService.java

@@ -23,7 +23,7 @@ public class SignUpService {
     @Transactional
     public Token signUp(final SignUpAccountRequest request) {
         UserJpaEntity user = doAccountStep(request);
-
+        log.info("User signed up successfully: {}", user);
         return authTokenManager.generateAuthToken(user);
     }