refactor(importing): inject FileStreamOpener to remove test-only seam

DocumentImporter exposed a package-private openFileStream(File) so a
Mockito spy could force the IO-error branch of isPdfMagicBytes. The
test-only seam leaked into production: the method existed for testing,
not for any production extensibility.

Replace with a constructor-injected FileStreamOpener interface (single
abstract method, @FunctionalInterface) and a one-line
@Component DefaultFileStreamOpener delegate. Tests now inject a mock
opener instead of spying on the importer itself, which is also a more
idiomatic Mockito usage.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/src/main/java/org/raddatz/familienarchiv/importing/DocumentImporter.java

@@ -24,7 +24,6 @@ import software.amazon.awssdk.services.s3.model.PutObjectRequest;
 import org.raddatz.familienarchiv.tag.TagService;
 
 import java.io.File;
-import java.io.FileInputStream;
 import java.io.IOException;
 import java.io.InputStream;
 import java.nio.file.Files;
@@ -79,6 +78,7 @@ public class DocumentImporter {
     private final TagService tagService;
     private final S3Client s3Client;
     private final ThumbnailAsyncRunner thumbnailAsyncRunner;
+    private final FileStreamOpener fileStreamOpener;
 
     @Value("${app.s3.bucket:familienarchiv}")
     private String bucketName;
@@ -349,13 +349,10 @@ public class DocumentImporter {
         return INDEX_PATTERN.matcher(index).matches();
     }
 
-    // package-private: a Mockito spy in tests can override to inject IOException
-    InputStream openFileStream(File file) throws IOException {
-        return new FileInputStream(file);
-    }
-
     private boolean isPdfMagicBytes(File file) throws IOException {
-        try (InputStream is = openFileStream(file)) {
+        // FileStreamOpener is injected so tests can stub a throwing implementation for the
+        // IO-error branch without spying on the importer itself.
+        try (InputStream is = fileStreamOpener.open(file)) {
             byte[] header = is.readNBytes(4);
             return header.length == 4
                     && header[0] == 0x25  // %

backend/src/main/java/org/raddatz/familienarchiv/importing/FileStreamOpener.java

@@ -0,0 +1,33 @@
+package org.raddatz.familienarchiv.importing;
+
+import org.springframework.stereotype.Component;
+
+import java.io.File;
+import java.io.FileInputStream;
+import java.io.IOException;
+import java.io.InputStream;
+
+/**
+ * Test seam for opening a {@link File} as an {@link InputStream}. Extracted so the magic-byte
+ * check in {@link DocumentImporter} can be unit-tested for the IO-error branch by injecting a
+ * mock that throws, without needing a Mockito spy on the importer itself.
+ *
+ * <p>Production uses {@link DefaultFileStreamOpener}, a one-line delegate to
+ * {@code new FileInputStream(file)}.
+ */
+@FunctionalInterface
+public interface FileStreamOpener {
+
+    /** Opens {@code file} for sequential reads. Caller closes the returned stream. */
+    InputStream open(File file) throws IOException;
+
+    /** Default production implementation: plain {@code FileInputStream}. */
+    @Component
+    final class DefaultFileStreamOpener implements FileStreamOpener {
+
+        @Override
+        public InputStream open(File file) throws IOException {
+            return new FileInputStream(file);
+        }
+    }
+}