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>

docker-compose.yml

@@ -72,6 +72,9 @@ services:
       - archive-net
 
   # --- OCR: Python microservice (Surya + Kraken) ---
+  # Single-node only: OCR training reloads the model in-process after each run.
+  # Running multiple replicas would cause training conflicts and model-state divergence.
+  # See ADR-001 for the architectural rationale.
   ocr-service:
     build:
       context: ./ocr-service