fix(ocr): use presigned URLs for MinIO access from OCR service

The OCR service was getting 403 Forbidden because it tried to
download PDFs from MinIO using plain internal URLs without
authentication. MinIO buckets are private.

- Add S3Presigner bean to MinioConfig
- FileService.generatePresignedUrl(): generates 15-min presigned URLs
- OcrService uses presigned URLs instead of plain internal URLs
- Remove unused s3InternalUrl / bucketName @Value fields from OcrService

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

backend/src/main/java/org/raddatz/familienarchiv/config/MinioConfig.java

@@ -5,6 +5,7 @@ import software.amazon.awssdk.auth.credentials.StaticCredentialsProvider;
 import software.amazon.awssdk.regions.Region;
 import software.amazon.awssdk.services.s3.S3Client;
 import software.amazon.awssdk.services.s3.S3Configuration;
+import software.amazon.awssdk.services.s3.presigner.S3Presigner;
 import org.springframework.beans.factory.annotation.Value;
 import org.springframework.boot.CommandLineRunner;
 import org.springframework.context.annotation.Bean;
@@ -44,6 +45,19 @@ public class MinioConfig {
                 .build();
     }
 
+    @Bean
+    public S3Presigner s3Presigner() {
+        return S3Presigner.builder()
+                .endpointOverride(URI.create(endpoint))
+                .serviceConfiguration(S3Configuration.builder()
+                        .pathStyleAccessEnabled(true)
+                        .build())
+                .region(Region.of(region))
+                .credentialsProvider(StaticCredentialsProvider.create(
+                        AwsBasicCredentials.create(accessKey, secretKey)))
+                .build();
+    }
+
     @Bean
     public CommandLineRunner testS3Connection(S3Client s3Client) {
         return args -> {