fix(async): revert to AbortPolicy — CallerRunsPolicy blocks requests

CallerRunsPolicy would cause the HTTP request to hang for minutes
if the queue is full. AbortPolicy with queue=100 is safe — the queue
will never realistically fill for a family archive. If it somehow
does, a clear error is better than a silent multi-minute hang.

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

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

@@ -20,7 +20,7 @@ public class AsyncConfig {
         executor.setMaxPoolSize(2);
         executor.setQueueCapacity(100);
         executor.setThreadNamePrefix("Async-");
-        executor.setRejectedExecutionHandler(new ThreadPoolExecutor.CallerRunsPolicy());
+        executor.setRejectedExecutionHandler(new ThreadPoolExecutor.AbortPolicy());
         return executor;
     }
 }