fix(async): increase thread pool to 2 threads + queue of 10

The old pool (1 thread, queue=1) meant OCR blocked all other async
tasks (imports). Now 2 concurrent async tasks with a queue of 10
— enough for OCR + import to run in parallel.

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

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

@@ -16,10 +16,10 @@ public class AsyncConfig {
     @Bean
     public Executor taskExecutor() {
         ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
-        executor.setCorePoolSize(1);
-        executor.setMaxPoolSize(1);
-        executor.setQueueCapacity(1);
-        executor.setThreadNamePrefix("Import-");
+        executor.setCorePoolSize(2);
+        executor.setMaxPoolSize(2);
+        executor.setQueueCapacity(10);
+        executor.setThreadNamePrefix("Async-");
         executor.setRejectedExecutionHandler(new ThreadPoolExecutor.AbortPolicy());
         return executor;
     }