feat(ocr): add Python OCR microservice, RestClientOcrClient, Docker Compose

Python microservice (ocr-service/):
- FastAPI app with /ocr and /health endpoints
- Surya engine: transformer-based OCR for typewritten/modern handwriting
- Kraken engine: historical HTR for Kurrent/Suetterlin with
  pure-Python polygon-to-quad approximation (gift wrapping + rotating calipers)
- Eager model loading at startup via lifespan context manager
- PDF download via httpx, page rendering via pypdfium2 at 300 DPI

Java RestClientOcrClient:
- Implements OcrClient + OcrHealthClient interfaces
- Calls Python service via Spring RestClient
- Health check with graceful fallback

Docker Compose:
- New ocr-service container (mem_limit 6g, no host ports)
- Health check with start_period 60s for model loading
- ocr_models volume for Kraken model files
- Backend depends on ocr-service health

Refs #226, #227

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

ocr-service/requirements.txt

@@ -0,0 +1,6 @@
+fastapi[standard]==0.115.6
+surya-ocr==0.6.3
+kraken==5.2.9
+pillow==11.1.0
+pypdfium2==4.30.0
+httpx==0.28.1