fix: remove plaintext password from startup log in DataInitializer

The log statement revealed the default admin password in application
logs. Now only the username is logged, using the resolved variable
instead of a hardcoded string.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/src/main/java/org/raddatz/familienarchiv/config/DataInitializer.java

@@ -61,7 +61,7 @@ public class DataInitializer {
                         .build();
                 userRepository.save(admin);
 
-                log.info("Default Admin erstellt: User='admin', Pass='admin123'");
+                log.info("Default Admin erstellt: User='{}'", adminUsername);
             }
         };
     }