fix(import): add @Schema annotations and fix IOException test coverage

- Add @Schema(requiredMode = REQUIRED) to SkippedFile and ImportStatus
  record components so TypeScript codegen produces non-optional fields
  when generate:api is next run
- Extract openFileStream(File) as package-private method so the
  IOException path can be tested deterministically without relying on
  OS-level file permissions (which are bypassed when running as root)
- Replace assumeTrue-based IOException test with Mockito spy that stubs
  openFileStream — test now runs in CI unconditionally (45 tests, 0 skipped)

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

backend/src/test/java/org/raddatz/familienarchiv/importing/MassImportServiceTest.java

@@ -40,7 +40,6 @@ import java.util.zip.ZipOutputStream;
 
 import static org.assertj.core.api.Assertions.assertThat;
 import static org.assertj.core.api.Assertions.assertThatThrownBy;
-import static org.junit.jupiter.api.Assumptions.assumeTrue;
 import static org.mockito.ArgumentMatchers.any;
 import static org.mockito.Mockito.*;
 
@@ -572,20 +571,19 @@ class MassImportServiceTest {
     @Test
     void runImportAsync_skipsFile_whenMagicBytesCheckThrowsIOException(@TempDir Path tempDir) throws Exception {
         Files.writeString(tempDir.resolve("unreadable.pdf"), "some content");
-        File unreadable = tempDir.resolve("unreadable.pdf").toFile();
         buildMinimalImportXlsx(tempDir, "unreadable.pdf");
         ReflectionTestUtils.setField(service, "importDir", tempDir.toString());
-        unreadable.setReadable(false);
-        assumeTrue(!unreadable.canRead(), "Requires non-root file permissions");
-        try {
-            service.runImportAsync();
-            assertThat(service.getStatus().skipped()).isEqualTo(1);
-            assertThat(service.getStatus().skippedFiles())
-                    .extracting(MassImportService.SkippedFile::reason)
-                    .containsExactly("FILE_READ_ERROR");
-        } finally {
-            unreadable.setReadable(true);
-        }
+        lenient().when(documentService.findByOriginalFilename(any())).thenReturn(Optional.empty());
+
+        MassImportService spyService = spy(service);
+        doThrow(new java.io.IOException("simulated read error")).when(spyService).openFileStream(any(File.class));
+
+        spyService.runImportAsync();
+
+        assertThat(spyService.getStatus().skipped()).isEqualTo(1);
+        assertThat(spyService.getStatus().skippedFiles())
+                .extracting(MassImportService.SkippedFile::reason)
+                .containsExactly("FILE_READ_ERROR");
     }
 
     // ─── readOds — XXE security regression ───────────────────────────────────