docs(ocr): document single-node constraint for OCR training

Training reloads the Kraken model in-process on the Python service.
The DB-level RUNNING constraint prevents concurrent API calls but
cannot protect against multi-replica deployments. Added explicit
comments in docker-compose.yml and OcrTrainingService to prevent
accidental horizontal scaling. See ADR-001.

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

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

@@ -42,6 +42,10 @@ public class OcrTrainingService {
             List<OcrTrainingRun> runs
     ) {}
 
+    // Not safe for horizontal scaling: training reloads the Kraken model in-process on the
+    // Python OCR service after each run. The DB-level RUNNING constraint (V30 partial unique
+    // index) prevents concurrent training API calls, but cannot prevent two OCR service replicas
+    // from diverging on model state. Deploy as a single instance only. See ADR-001.
     @Transactional
     public OcrTrainingRun triggerTraining(UUID triggeredBy) {
         if (trainingRunRepository.findFirstByStatus(TrainingStatus.RUNNING).isPresent()) {