fix(test): skip IOException test when running as root

setReadable(false) silently no-ops as root; check canRead() to guard
the assumption correctly so the test is skipped in Docker CI.

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

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

@@ -575,7 +575,8 @@ class MassImportServiceTest {
         File unreadable = tempDir.resolve("unreadable.pdf").toFile();
         buildMinimalImportXlsx(tempDir, "unreadable.pdf");
         ReflectionTestUtils.setField(service, "importDir", tempDir.toString());
-        assumeTrue(unreadable.setReadable(false), "Requires non-root file permissions");
+        unreadable.setReadable(false);
+        assumeTrue(!unreadable.canRead(), "Requires non-root file permissions");
         try {
             service.runImportAsync();
             assertThat(service.getStatus().skipped()).isEqualTo(1);