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>

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

@@ -0,0 +1,73 @@
+package org.raddatz.familienarchiv.service;
+
+import com.fasterxml.jackson.annotation.JsonProperty;
+import lombok.extern.slf4j.Slf4j;
+import org.raddatz.familienarchiv.model.ScriptType;
+import org.springframework.beans.factory.annotation.Value;
+import org.springframework.core.ParameterizedTypeReference;
+import org.springframework.http.MediaType;
+import org.springframework.stereotype.Component;
+import org.springframework.web.client.RestClient;
+
+import java.util.List;
+import java.util.Map;
+
+@Component
+@Slf4j
+public class RestClientOcrClient implements OcrClient, OcrHealthClient {
+
+    private final RestClient restClient;
+
+    public RestClientOcrClient(@Value("${app.ocr.base-url:http://ocr-service:8000}") String baseUrl) {
+        this.restClient = RestClient.builder().baseUrl(baseUrl).build();
+    }
+
+    @Override
+    public List<OcrBlockResult> extractBlocks(String pdfUrl, ScriptType scriptType) {
+        Map<String, String> body = Map.of(
+                "pdfUrl", pdfUrl,
+                "scriptType", scriptType.name(),
+                "language", "de");
+
+        List<OcrBlockJson> response = restClient.post()
+                .uri("/ocr")
+                .contentType(MediaType.APPLICATION_JSON)
+                .body(body)
+                .retrieve()
+                .body(new ParameterizedTypeReference<>() {});
+
+        if (response == null) return List.of();
+
+        return response.stream()
+                .map(OcrBlockJson::toResult)
+                .toList();
+    }
+
+    @Override
+    public boolean isHealthy() {
+        try {
+            restClient.get()
+                    .uri("/health")
+                    .retrieve()
+                    .toBodilessEntity();
+            return true;
+        } catch (Exception e) {
+            log.warn("OCR service health check failed: {}", e.getMessage());
+            return false;
+        }
+    }
+
+    record OcrBlockJson(
+            @JsonProperty("pageNumber") int pageNumber,
+            double x,
+            double y,
+            double width,
+            double height,
+            List<List<Double>> polygon,
+            String text
+    ) {
+        OcrBlockResult toResult() {
+            return new OcrBlockResult(pageNumber, x, y, width, height, polygon, text);
+        }
+    }
+}