feat(backend): add thumbnailExecutor bean for isolated thumbnail workload

Dedicated thread pool (core=1, max=2, queue=200) with CallerRunsPolicy
for back-pressure. Keeps thumbnail rendering off the shared taskExecutor
used by OCR and out of the AbortPolicy queue that drops work on overflow.
Quick-upload batches (15+ files) now apply back-pressure instead of
silently dropping thumbnail jobs.

Refs #307

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

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

@@ -37,4 +37,19 @@ public class AsyncConfig {
         executor.setRejectedExecutionHandler(new ThreadPoolExecutor.AbortPolicy());
         return executor;
     }
+
+    @Bean("thumbnailExecutor")
+    public Executor thumbnailExecutor() {
+        ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
+        executor.setCorePoolSize(1);
+        executor.setMaxPoolSize(2);
+        executor.setQueueCapacity(200);
+        executor.setThreadNamePrefix("Thumbnail-");
+        // CallerRunsPolicy applies back-pressure to quick-upload batches and admin backfill
+        // instead of dropping work (shared taskExecutor uses AbortPolicy). Safe because the
+        // task is dispatched via TransactionSynchronization.afterCommit, which runs on a
+        // post-commit callback thread without active transaction synchronization.
+        executor.setRejectedExecutionHandler(new ThreadPoolExecutor.CallerRunsPolicy());
+        return executor;
+    }
 }