fix(async): queue 100 tasks + CallerRunsPolicy instead of abort

Better to wait than to error. Queue capacity 100 holds plenty of
OCR jobs. CallerRunsPolicy means if the queue is somehow full,
the request blocks instead of getting rejected with an exception.

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

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

@@ -18,9 +18,9 @@ public class AsyncConfig {
         ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
         executor.setCorePoolSize(2);
         executor.setMaxPoolSize(2);
-        executor.setQueueCapacity(10);
+        executor.setQueueCapacity(100);
         executor.setThreadNamePrefix("Async-");
-        executor.setRejectedExecutionHandler(new ThreadPoolExecutor.AbortPolicy());
+        executor.setRejectedExecutionHandler(new ThreadPoolExecutor.CallerRunsPolicy());
         return executor;
     }
 }