docs(ocr): document tail-recursive queue drain design in promoteNextQueuedRun

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

backend/src/main/java/org/raddatz/familienarchiv/service/SenderModelService.java

@@ -173,6 +173,11 @@ public class SenderModelService {
      * Promotes the oldest QUEUED sender run to RUNNING and triggers its training.
      * Called in the finally block of triggerSenderTraining, creating a sequential chain:
      * each run promotes the next only after it fully completes (success or failure).
+     *
+     * This is intentionally tail-recursive via the @Async thread: the same thread holds the
+     * full queue drain, serialising all sender training runs naturally without an external
+     * scheduler. With N queued runs the thread stays occupied for N sequential trainings —
+     * acceptable because the @Async executor is dedicated to long-running background work.
      */
     private void promoteNextQueuedRun() {
         Optional<OcrTrainingRun> queuedOpt = txTemplate.execute(status ->