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/models.py

@@ -0,0 +1,20 @@
+from pydantic import BaseModel, Field
+
+
+class OcrRequest(BaseModel):
+    pdf_url: str = Field(..., alias="pdfUrl")
+    script_type: str = Field("UNKNOWN", alias="scriptType")
+    language: str = "de"
+
+
+class OcrBlock(BaseModel):
+    page_number: int = Field(..., alias="pageNumber")
+    x: float
+    y: float
+    width: float
+    height: float
+    polygon: list[list[float]] | None = None
+    text: str
+
+    class Config:
+        populate_by_name = True